"Delivered by Tuesday" is a promise the store writes and the courier keeps — or doesn't. Write it too tight and the buyer gets a broken promise; too loose and they buy from whoever promised sooner. The promise deserves better than a guess: it's measurable, per lane, from your own data.
Both errors cost — differently
Overpromising is the trust killer: a 2-day promise on a 4-day lane produces a WISMO ticket, a disappointed buyer and a broken contract — even though the parcel arrived "on time" by the courier's SLA. Underpromising is the silent cost: "7–10 days" on a lane that delivers in 3 suppresses conversion — buyers who needed it sooner bounce, and the ones who stay grade the 3-day delivery as "expected", not impressive.
The measured-promise method
Your own shipment history is the dataset: group deliveries by PIN prefix and measure actual days-to-delivered per lane. Promise at the P90 — the transit time that 90% of parcels beat — framed as a range ("Thu–Fri", not "Thu"). The tail ten percent still overruns, but as an exception the support team can handle, not a breach of contract.
Two refinements: per-carrier where it differs — Delhivery's Jaipur transit may beat your blended number by a day; and festive overlay — apply a temporary buffer in known crunch weeks rather than let December teach the same lesson it teaches every year.
A delivery promise isn't marketing copy. It's a forecast — and forecasts are only honest with data behind them.
The bottom line
The credible promise is the measured one. Count actual transit days per lane, promise at P90 as a range, and the store stops breaking contracts it never meant to write — and stops losing buyers to promises it could have kept.
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