Not really an AIBU, as under the circumstances I think I should report them instead of giving the benefit of the doubt as it has happened multiple times.
A small independent corner store has recently opened near my mum in the last month. The first time I shopped there for a few bits I had my newborn with me and was rushing to get home to feed him. I didn't notice until after that they overcharged me 66p.
This has happened to my relatives each time they have purchased from the store for varying amounts. Sometimes it's 10p, 4p or a pound. My siblings have started querying the total when it has been more noticeable and the cashier corrected it. In total this has happened to about five of us.
This happened again to me today when I popped into the store for a few bits. I noticed they were trying to overcharge me by a pound and they corrected this when questioned the total. They also don't give the receipts and quickly dispose of it after payment.
Would I be unreasonable to report them without at least mentioning to them first that they have overcharged on multiple occasions?