Hi,
I'm coming to the US pretty soon for a little trip over east coast:
EWR-FLL United
MIA-IAD AA
IAD-YTZ Porter
YYZ-EWR Air Canada

The problem is that I'm bringing a checked baggage, and as you have probably already guessed I don't really care about FF programs, so I have to pay 25$ per leg to take it with me. However, looking through bookings management options I haven't found any way to pay those fees (fact that I have booked tickets via travel agent may also play a role). Will they become available at online checkins? Or can I pay them at check-ins counters at airports? (if so, are credit cards allowed or cash only?)

This may sound a bit stupid for many of you, sorry about that, but most European airlines I used to don't charge baggage fees by default, and even if they do rules are usually much clearer.