Mobile App Check In

[GoofyforDisney3]

I just used Delta's iPhone app to check in for my flight tomorrow and although I put that I was checking a bag, it didn't ask me for payment. Does Delta automatically charge it to the card I used when I bought the tickets or will I need to pay tomorrow when I check my bag?
Just want to make sure I don't have a problem on the trip home when I use RAC to check in.