You say you're in the glorious "Land O' Thaiz" now CanadaSam; what type of visa are you currently on here?
Your first step in getting a "yearly extension of stay" based on being over 50 (retirement) is getting a 90 day single entry Non-Immigrant Type O <-(other) visa. Then you can apply for your yearly extension of stay.
Now many agencies can pull off getting the Non-O & the yearly extension of stay in a few days. None that I know of can pull it all off in a single visit and in fact MOST of the agencies here in Bangkok pull the 90 day Non-O here at Chaengwattana, then courier your passport to Jomtien and pull the yearly extension of stay there.
Of course since they went to Zone Immigration's; getting a yearly extension of stay from an immigration office in a zone where you don't physically live is against the rules, but it doesn't seem to stop people from using the agencies.
I'll relate how 9 out of 10 agencies work. You show up at a thai bank, an 'rag head, red-dot' indian shows up with a bag of cash. They open an account for you, deposit the requisite 800K, get the bank manager to write a letter stating you have the money in your account, they update the bank book, they pull the money out, and they immediately ferry you to the thai immigration office at Chaengwattana to apply for the 90 day Non-O visa and the accompanying 90 day admitted until stamp.
Now most people who apply for a 90 day Non-O visa at Chaengwattana based on being over 50 must show that the money originated from overseas AND also have their applications go "under-consideration" for 14 days before you go back to get the Non-O inked into your passport. The visa "agencies" have "greased the wheel" and can get them issued on the spot, without the need of the funds originating from overseas & without the 14 day waiting period.
Additionally; when you apply for a yearly extension of stay you must have 45 days or LESS remaining on your current admitted until stamp. The agencies can also circumvent this by taking your passport to Jomtien where they're pulling the yearly extension of stay via the back door by also "greasing the wheel" down there. This means once you go thru them you'll have the 90 day Non-O and the yearly extension of stay which equals out to about 15 months in country.
You can run into problems when you go to do a 90 day report, but the officer sees you pulled your yearly extension from another zone. Filing a change of address gets you around this, BUT suppose the officer asks where you lived in Jomtien? What are you gonna tell them? You got NO idea what address the agency used to pull your yearly extension of stay down there!
I am not saying that agencies don't work, I'm saying there is a definite risk involved, even though they're real honest to goodness immigration stamps and signatures..
Case in point; there was a "famous" agency that was pulling Non-O's and yearly extensions based on being over 50 (retirement) from the Korat Immigration office. They specialized in getting them for Nigerians & Arabs. The officer was caught issuing visas/extensions via the back door up there and all the extension's they'd done for a year were canceled without telling the passport holders. The people who held these bogus extension stamps didn't know until they went to exit the country, OR to go apply for a new extension. It caused quite a few of them to come off the rails, get locked into IDC and be deported.
Another one that was in the news was; the thai immigration dept raided a bunch of hotels, guest houses down in Pattaya famous for it's indian clientele, It was found that 10-15 people had all used the same address when they got their extensions (the address the visa agency put on their passport) but that was not where those people were actually living, and they were fined for failure to file a change of address AND failure to notify the immigration office within 24 hours of moving AND upon further investigation, their paperwork was incomplete and some were also arrested for having a "fake" visa stamp (a real stamp gotten under false pretenses) and deported.
I'm not sayin' don't use an agency, I'm saying there's no reason you can't actually follow the rules and get both a Non-O and a yearly extension of stay all on your own. There's no requirement that the 800K baht ($23K-USD) you bank for 2 months prior to your application stay in your account for even a second after you get your extension. You could drain the account back to zero right after you have your extension inked into your passport.
I go out to the thai immigration office here in Bangkok at Chaengwattana A LOT (probably more than 300 times in 6 years) <-ONLY because I like the food court there, NOT because I 'push-paper'
I see the same agents, the same rag-head, red-dot indians inside the office and down in the banks time after time. They're certainly NOT hiding what they're doing.
Sorry this was a long post, perhaps you'll find it of marginal value