Easyjet - We checked in quite late, about a week before travelling Sat 8th. I never pay. It's never a problem.
Our seats are
17E, 16F, 17F, 16E, 18C, 16D, 18B, 15F, 18A, 15E, 18F
Return
39F, 38B, 38E, 38F, 39A, 39B, 39C, 39D, 39E, 38C, 38D
That would have cost me £287 if I'd have paid.