How can a US citizen extend their 30-day visa exemption while residing near Samrong BTS in Thailand?

Happy trails :)

Good. Not too far off with "a few extra thou's" then :)

Not sure how it works, but I think there's a fair chance you'll be fined when you process your application. I forget the amount (hopefully someone here can enlighten). But make sure you have a few extra thousand with you.

Yep, Samut Prakan is pretty set about you or your land lord having filed a TM.30 to use that office for extensions.

Did you also get fined?

That sounds really annoying. Thanks for updating.

Then to answer your original question, Samut Prakan is the office you should go to, since you live in Samut Prakan. Your proof of address should be sufficient (if that's even needed -it may be, i did actually search the web but cannot find a conclusive answer). But logically, should it be required, it would not have to cover the full 30 days, since you are planning to exit anyway, so it should be accepted.

If you encounter any issues with Samut Prakan, which I don't think you will, use Chaengwatthana as a second option, but be aware you need to put down on the application form a Bangkok address, could be anything, a hotel, a friend's, etc. You do not need to supply any proof for this address at Chaengwatthana. Just do not complete a Samut Prakan address at Chaengwatthana if you end up going there, because they will send you to Samut Prakan.

I seem to recall them being less user friendly than CW.

Technically, you should extend in Samut Prakan. Practically, some people would go Chaengwatthana because they require nothing else but the form, pic and money. But they would write down some Bangkok address on the form. Just not sure if Samut Prakan requires proof of address, frankly I am lazy to search ThaiVisa for this now, the OP could do this as well.

Oh! Damn. Good point.

I used the danish embassy and, I think it was somewhere between 1 and 2 k baht. Probably different in other embassies.

You can indeed. I got a "letter of reference" from my embassy. Really made it easier. Used Bangkok Bank at EmQuartier (Phrom Pong bts).