I'm sure that you've occasionally ran into cashiers who couldn't do simple arithmetic in their heads, I have too, but most employers who hire people to handle their money at least try to pick people who are at an adequate level.

I was always the guy who was 'bad at math' but if you have 6th grade math nailed you can (with the help of the POS, the cash register) get by fairly easily.

As /u/Raider61 pointed out, even if I knew off the top of my head that the change was a $10, I would have still typed in $21.18 into the till, because that way I know for sure I haven't made a mistake even if I'm on hour 10 of an 8 hour shift.

The 'giving extra money to get a whole dollar amount of change' thing is extremely common, so unless the it's the clerks first day, or they are really bad at math they probably aren't going to be confused by people giving them 'too much money'.