But I volunteer at a charity resale shop, and I'm one of the few people who isn't scared to run the register and make change while the impatient shoppers wait. Seriously, people suck. But I digress. Our register has buttons to signify cash in the most typical set amounts ($20, $40, $60, $100...there may be one or 2 others). 90%+ of cash-payers are giving us $20 bills anyway, so you hit one button, out pops the drawer with the correct change calculated, and Mr. or Ms. Impatient can be on their way. Putting in a custom amount is harder, because it's designed to be an exception, and it requires more time to actually do the transaction. Hit the "custom amount button", pull up the number keypad, key in the numbers, hit enter, and confirm the amount one more time.

I usually don't bother with the latter because it's such a pain in the ass, and besides...I've had sufficient education to be able to determine appropriate change and sufficient years on this planet not to give a fuck if I screw it up. But the most important thing is I'm not going to have anyone breathing down my neck for taking too long or if my pennies don't balance out at the end of my shift.

All that to say, it's entirely possible she was doing exactly what she had been trained/was supposed to do when handed an illogical amount based on the transaction. Or she might have been worried about taking too long to figure out how to do what you wanted.