What to Promise a Customer When You Don't Know When Stock Lands

Why an honest date beats a fast one, and how to give a date you can actually keep.

The phone rings. It's one of your best customers.

"Can I have 300 by Thursday?"

You've got 80 on the shelf. The rest is on a delivery that's due on Monday. It was supposed to arrive last Wednesday.

Every instinct says: say yes. Say Thursday. Let future you sort it out. Future you is always so capable.

This is how good customers are lost. Not by being slow, but by promising a date and missing it.

This is the last piece in our series on planning for the unpredictable. The first, when the lead time becomes a range, is about supplier dates that keep moving. The second, why every department gives a different answer to how much stock you have, is about seeing all your stock, including what's on the way. This one is about the moment both problems land on a customer's desk.

The short version. When you don't know exactly when stock will arrive, promise the date you can keep, not the date you hope for. Start with what's free on the shelf. If that's not enough, promise from the next delivery plus that supplier's usual delay. Give a range and a day you'll confirm by. Ship what you have now. Tell the customer before they have to chase. And decide who gets scarce stock before you run short, not during.

Does a faster promise really win the order?

Less than you'd think.

In a 2025 study, Kinshuk Jerath and Shin Oblander worked with a business-to-business online seller to see how the promised delivery time changed whether buyers bought. On average, each day taken off the promise lifted demand by 1.82%, about the same as a 2.21% discount.

But here's the interesting part. For promises under a week, buyers barely reacted. It was only past a week that demand dropped quickly.

In other words, most customers aren't demanding tomorrow. They want a reasonable date. And they want it to be true.

What does a missed promise really cost?

More than the order.

In a 2022 study using data from the online retailer JD.com, Nooshin Salari, Sheng Liu and Zuo-Jun Max Shen built a more accurate way of predicting and promising delivery dates. In their simulation, simply promising more accurately lifted sales by 6.1%. Nothing got delivered faster. The dates just got truer.

Then there's the cost nobody puts in a spreadsheet. A missed date quietly promotes your customer to private investigator. They call sales. They call the warehouse. Next time, they order early and order extra, just in case, because your dates have taught them to.

An honest late date builds trust. A hopeful early date spends it.

Why do customers over-order when stock is tight?

Because they've learned it's how they get their share.

The researchers who named the bullwhip effect in 1997, Hau Lee, V. Padmanabhan and Seungjin Whang, called it the rationing game. When stock is short and a supplier shares it out based on how much each buyer ordered, everyone has a reason to order more than they need, to get a bigger slice. Buyers even do it when they only expect a shortage. The result is a pile of inflated orders that makes the shortage look worse than it is.

Their fix was simple: share out scarce stock based on what customers actually bought before, not on what they ordered this week, and keep customers informed so they don't feel the need to play games.

So decide your rule before the shortage, not during it. First come, first served, past purchases, customer tier, or a fair share for everyone: pick one, write it down and tell your sales team. Otherwise stock goes to whoever shouts loudest or orders biggest, and you've accidentally built a loyalty programme for shouting.

How do you set a date you can actually keep?

Six steps, no new software needed.

1. Start with what's free. Not what's on the shelf, what's free: the stock in the building minus what's already promised to other orders. If that covers it, promise today's dispatch with confidence.

2. If not, promise from the next delivery, plus a buffer. Take the date the next delivery is due, then add the delay that supplier usually runs to. If their shipments tend to land four days late, your promise should already include those four days.

3. Give a range and a confirm-by date. "Between the 12th and the 16th, and we'll confirm the exact day by the 10th" beats a single date you're not sure of. The customer can plan, and you get time to check.

4. Ship what you have. Offer the 80 now and the rest when it lands. Many customers would rather have part of an order on time than all of it late. Just ask.

5. Tell them before they chase. The moment a delivery date moves, update every promise that depended on it, and tell the customer. "It's moved to the 18th" keeps trust. A customer finding out on the 16th, from an empty loading bay, does not.

6. Put the rules in the system, not in someone's head. Who gets priority, who gets scarce stock and how much buffer to add should be the same whoever answers the phone.

Who should own the promise?

Whoever makes it should be able to see what keeps it.

In a lot of businesses, sales make the promise, the warehouse keeps it, and purchasing decides whether it was ever possible. Each sees a different number. So sales promise from the shelf count without knowing half of it is already taken. Or they won't promise at all, because nobody can tell them when the next delivery is due.

The fix isn't another meeting. It's the same stock picture on every screen: what's free, who it's promised to, and what's coming and when.

How this runs on one platform

A promise you can keep needs free stock, promised stock and incoming stock in one place.

In Depot WMS, stock on the shelf can be traced to the sales orders it's reserved for, and each open purchase order shows as stock on the way, with the date it's expected. So sales can see at a glance what's genuinely free and what's coming.

When stock isn't there yet, Flow OMS takes the order as a backorder instead of turning the customer away. When the stock is received in Depot WMS, waiting orders are matched to it and fulfilment starts automatically. A workflow can tell the customer when their order moves from waiting to processing, and alert purchasing when too many orders are waiting on one product.

Flow OMS can also ship part of an order now and the rest when it arrives, all under the same order. Each business sets its own priorities, from express orders to customer tiers, and those priorities follow the order everywhere, so your best customers are handled the way you decided, every time.

Customers can follow their orders in a Kart eCom portal, with the status updating live. And for the last mile, the Cargo TMS tracking page shows a live arrival time based on where the driver actually is.

None of that makes the stock arrive sooner. It makes sure every promise is based on what you can actually see.

The short version

When you don't know when stock will land, the worst promise is the one you're hoping to keep. Promise from what's free, then from the next delivery plus a buffer. Give a range and a confirm-by date. Ship what you have. Update customers before they chase. And decide who gets scarce stock before you run short.

Customers forgive late. They don't forgive surprised.

Talk to us about making promises your operation can keep

Frequently asked questions

What delivery date should you promise when stock is on backorder?

Promise from the date the next delivery is due plus the delay that supplier usually runs to, not from the purchase order date alone. Give the customer a range and a firm date by which you will confirm the exact day. If some stock is available now, offer to ship it and send the rest when it arrives.

Does a faster delivery promise increase sales in B2B?

Less than many expect. A 2025 study of a B2B e-commerce company by Jerath and Oblander found buyers barely reacted to promised delivery times under a week, but demand dropped quickly beyond a week. On average, each day removed from the promise raised demand by 1.82%, similar to a 2.21% discount.

Is it better to promise a fast date or an accurate one?

An accurate one. Missed dates cost trust and lead customers to over-order and chase. A 2022 study by Salari, Liu and Shen using data from JD.com found that a more accurate way of promising delivery dates lifted sales by 6.1% in simulation, without any delivery getting faster.

How should scarce stock be shared between customers?

By a rule decided before the shortage, such as past purchases, order date or customer tier. Sharing stock in proportion to the size of current orders encourages customers to inflate their orders, which Lee, Padmanabhan and Whang called the rationing game in their 1997 study of the bullwhip effect.

What is available to promise?

Available to promise is the amount of stock you can commit to new customer orders. It is the stock on hand, minus stock already promised to other orders, plus stock due to arrive in time for the date the customer wants.

References

Jerath, K. and Oblander, S. (2025). The Nonlinear Impact of Promised Delivery Time on Online Purchasing. Manufacturing & Service Operations Management.

Salari, N., Liu, S. and Shen, Z.-J. M. (2022). Real-Time Delivery Time Forecasting and Promising in Online Retailing: When Will Your Package Arrive? Manufacturing & Service Operations Management, 24(3), 1421-1436.

Lee, H. L., Padmanabhan, V. and Whang, S. (1997). Information Distortion in a Supply Chain: The Bullwhip Effect. Management Science, 43(4), 546-558.

More in this series

This piece closes Planning for the Unpredictable. Start with when the lead time becomes a range, then why every department gives a different answer to how much stock you have.

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