If you have Bangkok Bank account see if your home country has a branch and do the payments through them.
I used to do that a BB branch in London. Transfer from my normal bank to them in GBP and they then did the Intl transfer and exchange rate which was a good rate
Some cracking deals with Chinese airlines if your willing to entertain them. Cannot comment on quality of seats or service as never used them but recently on my searches price wise cannot be beaten
Go onto the Goverment website it is free of charge. Took me 5 to 10 mins, when I arrived at Thailand I had a printed copy and an electronic copy on my expecting to have to show it. The immigration officer asked for neither he just checked my boarding pass and passport and let me in so I take from that it all recorded in their system.
A lot of varying stories and experiences, at the end of the day its down to your luck with the Immigration Officer you get. I come 3 or 4 times a year for 26 days on visa excemption I have sometimes been asked intended length of stay but I arrived on Sat 12th and no questions just stamped in and wished a good stay
I just googled TDAC Thailand clicked on the lunk and five minutes later with no charge had my approval. Will find out tomorrow when I arrive in swampy if I did it right lol