Thai Air Asia also has three cheap flights a day to Krabi (733) but unfortunately also all before 18hrs.
One-Two-Go almost always do their domestic flights with the increasing MD
-80 fleet. The 747s mainly go to Hong Kong, Korea etc. and for rare domestic bulk transport during Thai New Year etc. but you should expect an MD
-80 in 99% of the cases.
Also Nok Air fly to Krabi.
Be very careful were the low costs depart. Don Muang (One two go) is the old airport... calculate at least 2 hours and 600 baht for the taxi ride from the new airport (Suvarnabhumbi or so) to there. The low cost airlines usually fly from there, so maybe better take Thai Airways. Air Asia also still seems to fly from Suv/BKK
but please get yourself confirmed on that.
All their websites are pretty good and bookable with western Creditcards