I'm not sure when you mean....before you head to WDW, for your departure flight from home? Or once you actually get to WDW, for your flight back home?
The night before you checkout, you will get a notice as to when to report to the DME bus stop. It will also give you a number to call in order to pay for checked bag fees...that would be a line to BAGS.
But, if you want to pay now, before you even leave home? I doubt it.