Might Be Counterfeit
There is only one place in the world where you’re allowed to pay the service providers in ones, but we’re not going to get into that. According to the OP, they were helping a customer out with a pretty large purchase, and, for some reason, the customer thought it would be humorous to pay in all single-dollar bills.
Well, that’s not the punchline. The real punchline is that the OP examined every single dollar bill under a blacklight to make sure they weren’t counterfeit. They didn’t disclose how many bills they checked, but we guarantee the Karen was fuming by the time the OP was done.