Does EasyJet charge more for additional luggage or seat selection if I do it after booking? Looking to travel with them in September, so still far out to know how much luggage I should pre-pay. Either way, I would definitely be avoiding adding luggage last-minute at the airport, for sure.

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!

Yes, this is the business model of European LLC (Ryanair, Wizz, Vueling etc). Adding luggage post-booking is more expensive (but often the difference is not significant). And yes, I believe with most LLC the luggage add-on price fluctuates a bit (i.e. one day you will find one price, the next day it will be slightly higher or lower).