Just for future reference:

Since there's no discount for paying on-line, no advantage. Since you have to interact with the kiosk to check bags anyway, swiping CC there produces a receipt printed by the kiosk (done that on occasion also).

Odd that you didn't get an email receipt though - I'd ask the agent when you hand over the bag if they can get you a receipt - maybe they can help.

Since my travel isn't business - my reference to needing one comes from the needing one in case the computer lost the payment record - Forgot that most people travel for work - sorry if I sounded snide