@Luc ***********
The "rules" are written in Thai by halfwits and then badly translated into English and other languages.
The same "rules" say that you can simply extend at immigration for Bt1900 without having to show further documentation but that law (yes, real law) is being abused by immigration.
As ever, This Is Thailand and applying from your home country and not somewhere you just bounce to for 5 days is going to be the safest option.