rule …. No such thing in Thailand. Extend and Border run as long as you can get away with it is the rule in Thailand: you will know as you will be warned ( “this is last time” the immigration officer will tell you, usually they do not refuse entry….). They however can start looking at other criteria which could be used against you: exit route out of the country within 90 days, 20,000 baht on you, booked accommodation so being compliant after few extension and border runs is crucial.
the letter i am picking up on Monday addressed to only one bank i had to nominate. The letter however free (but it takes 5 working days). Different IO do things differently.
and a letter of residency from immigration - Some immigration offices charge 100 baht and some 0 baht but the letter is addressed and valid for one bank only (Kasikorn etc)