How easy is it to implement a BOGOF (buy one get one free) or 3 for 2 , or buy one get one half price on an invoice in access?

Would this involve a separate "Offers Table"? and how would one go about the calculations?

I'm thinking on the lines of INT to round down after dividing the quantity by the value of the specific offer in the table.

for example, if a 3 for 2...

Check quantity in this example 5 in total.
5 / 3 rounded down is 1 so qualifies (with a remainder of 2)
price per qty x 2 (for the three) + the 2 qty each = qty x 4

Probably long winded but hopefully you get where I'm heading.

Was wondering if there was an easier way or if someone has already done this? if so.. how'd you go about it?