I don't normally check bags on easyJet but I have noticed that prices for bags and seats often vary if you add these at a later stage. There's not normally a significant difference, perhaps a few GBPs per bag. As you suggest, paying at the airport is the most expensive option.

Low cost airlines are are generally sticklers regarding bag sizes and weight, and easyJet is no different. I would ensure your carry on allowance (whatever that might be - some fares only permit one free smaller bag to go under the seat in front of you) conforms with your allowance because if you turn up at the gate with luggage that's too big (or too many) then any bag that falls outside the strict allowance will be gate-checked at £48 per bag!