I noticed a recent bill from Opera restaurant added a 'rounding' fee. They rounded the bill up to the next whole QR. This was not posted on the menu or in the store.

If there is a coin shortage, then the store should just post a sign at the entrance saying this is the store policy - no change or gum/candy as change (and the rounding up or down rules). If you enter the store, then you agree to this policy. Otherwise, people assume you are trying to scam them out of money.