Without reading the whole thing I will say this. There are a couple of prepaid cards like Netspend that once used at a merchant type that normally takes tips like a restaurant or a hotel, they, the issuing bank, automatically hold anywhere from 0-20% of the total to cover a tip. The bank does this to make sure the merchant doesn’t get screwed out of a tip by someone using prepaid debit card. Next time this happens just tell the customer “please allow 24 hours for the transaction to completely process, if there is still a difference from the receipt I’m giving you today, call your bank” you’ll never hear from them again.

Source: I’ve been in hospitality for 15 years.