You clearly have no idea what you're talking about.
First of all : extensions are standard 1900 baht : non-O's , tourist visas, exempt,...
Second : there is no 15 month multiple entry retirement visa : that doesn't exist.
Probably (my guess) you are talking about a non-O visa based on being at least 50 (commonly called retirement visa) which is 3 months --> 2000 baht. THEN a year extension --> 1900 baht (Which is separate, and not even a visa but an extension) those are NOT even multiple entry visa or extensions. So then you have to buy a multiple entry permit separately to get that effect --> 3800 baht or 4000 if you buy it at the airport without picture and form.
And this wouldn't even give you multiple entry on your complete 15 months by the way.
And explain me how doing it yourself directly with immigration you would suddenly end up with a "dodgy visa
Extra info : happened 29 November 2023 (so 2 and a half months ago). Officer kept the documents. He had to come back next day with bankbook (updated it bank downstairs at immigration first), could cut the queue straight to the same officer as the day before.
It doesn't matter how many times it hasn't been checked. What counts is that as soon as there is an instance when it DID happen, from then on you have to take into account that an official at Jaeng Watthana immigration MAY require your bankbook for the transfer of stamps.
I know of a 100% true case a few months ago where he had to come back the next day with the bankbook (but luckily could cut the queue).
This also means that anyone who used an agent to circumvent the money requirements suddenly gets into unforeseen problems.
Passport number is not always on driving license. It depends. But probably majority of foreigners will be passport number. But can also be the Thai id number.