My recent reply to that was "hang on while I lot on to my phone app and check."
I'd have to find the actual till receipt, not being an 'app' sort of person. And it would still be in the car. But they'd wait, surely... :)
The autistic comment made me laugh. My latest receipts are scrunched up & being batted about the floor by the kitten. I'll have to retrieve it for the stash for the burny bin where all of our non-important receipts go after six months.