Can I give a receipt if a customer paid cash?
Certainly. Do you have a printer? If so, you should be prompted to print a receipt after a transaction. If you don't use a printer, you should be able to enter a text number or email address to send a receipt for a cash sale just you can for a credit card sale. If the option doesn't come right up for some reason, you can pull up the transaction in Activity and choose Issue Receipt. There you can print a receipt if you have a printer or enter the information required to send one to the customer.
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