I fly Southwest Airlines. I avoid the Sky Captain upfront. It is my understanding that these folks are separate from Southwest Airlines. The one time that I used them (and did not know) they demanded a tip. Southwest Airlines (inside) are professional. I'm sure that they would accept cash for extra/overweight luggage, but a credit card would protect you. Good luck!
I used southwest and was able to get change from their register to tip my skycap I was allowed to check 2 luggage up to 50lbs for under the plane and a carry on plus my purse for no additional charges only tip. If you have more than that or your airline charges for luggage they should be able to take cash
I'm not sure but my first suggestion would be to check in with the sky captain in the front before entering the terminals! : ) good luck!!
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