How to enforce fixed quantity multiplier in self service procurement?

Summary:

Client has a need to be able to enforce fixed multiplier for expense items requested through self service procurement so that the requester is not able to put a quantity into the cart if it does not meet the fixed multiplier.

Example:

There is an agreement line covering soft drink in pacs of 24 cans. Client wants to be able to restrict requesters to order in full pacs of 24 cans. The desired functionality is that if a requester tries to order 1.5 pacs (equal to 36 cans) then she will get an on screen error message saying that only multiple of 1 pack is allowed.