It's been 20 years since someone showed up at my house and slept on the sofa for a while but in that period visited Sainsbury's and discovered that their implementation of "Buy one get one free" was to remove the price of one object from the total, and the implementation of "Reduced price" was to charge the full price and then remove a chunk and the combination was buy two objects, have both reduced in price, and then deduct the full price of one object
@mjg59 I know someone who worked for Tesco and they had the exact same issue many years ago. I wonder how much software they have in common.
@jmacarthur @mjg59 Co-op supermarket had the same issue circa 2001, I remember getting a trolly full of carrots in order to bring a fifteen pound shop down to under aa pound as a student. We donated the carrots to the equestrian campus