7kg carry on (but they didn't really check mine), and 20kg it's extra for 500thb. I ended up shipping it via Kerry Express to my address in BKK, for about 400thb (it was actually 75kg worth in 3 bags, easier to just have it delivered at my door vs me carrying in the taxi).
You can just go to AirAsia's website and go through the booking process, you'll be able to see the prices of checked luggage options (20,25,30kg etc).
Make sure it’s SHA Plus + not SHA+ . But nothing needed just proof of the reservation when you upload to Thai pass , probably helps if the reservation mentions it.