There's lots of stuff about what I do that doesn't make much sense :)

It works in this scenario because the stacks are reliably sorted by customer and date, and each form has a running tally of what cookies are on offer as things get added to the list.

Assume customer x's forms are taken out, and you make two stacks of them without shuffling the forms. The very first form on the first stack from 2022-01-01 does not include cookie y. The first form on the second stack, from 2023-02-01, also does not contain cookie y. Based on this information and the conditions above, you can infer that the form you want is in the second stack.

Now, if the forms were not reliably sorted, or did not contain a running record, you'd need to approach this differently. Strategies would probably involve inferences or straight getting the info you need from other sources - custumer correspondence around "We want cookie y, how much?" (if it occurred when you were in a position to get such correspondence); knowledge of big changes to cookie offerings to the customer (contract renewals); bugging accounting at a regular, annoying cadence with progressive escalation until they answer/complain about you bugging them, etc.