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Is it bad etiquette to use a credit or debit card to pay for single figure amounts at the POS
Generally, I consider it bad etiquette to inconvenience others.   I would recommend cash for small purchases. Try to offer as close to the required amount as possible.  Don't pay with several dollars worth of change if you can avoid it. You shouldn't need to carry a lot of cash.  When you do don't make it obvious.