Hi @astriping,
Great question! I think I can shed some light on what might have happened here. Sales receipts are typically used to take a payment instantly, while invoices can be e-mailed to a customer for them to pay at a later time. When you created the sales receipt, if you have the customer's credit card information saved and the process credit card button is checked, it will process a payment.
Here's a screenshot showing that button:
If you uncheck the button, it won't process the payment when you save it.
I hope that helps!
Lucas