You can add it anytime until 3 hours before the flight online, after that you pay at checking (double the price). Also, I've flown them several times in the last few months, TLV, BUD, WAW, and LTN and only at Luton airport were they very strict about checking the sizes of the large cabin bags. At the other airports as long as it's not egregiously over their size limits they were ok with it. Also at Luton they strictly enforce the one bag only limit.