Why a screenshot proves nothing
A transfer alert is an image. Images are editable, forwardable and reusable, and a pending transfer looks identical to a settled one. Every business here has released goods against one and learned this the expensive way.
Checking your own bank app is better, but it does not scale past a few orders an hour, and at a busy counter or a customer's gate nobody is doing it properly.
What an account number per order changes
If each order has its own account number, a payment landing in it identifies itself: that customer, that order, that amount. Nothing has to be matched by hand and nobody has to judge an image.
The rider at the door never has to decide. Their screen turns green when the bank says the money arrived, and until then the goods stay on the bike.
What about part-payments and late payments
A part-payment shows as exactly that, with the balance visible to both of you. A payment arriving after the account number expires still finds the order — you decide whether to proceed or refund.