@And***
if you live far away from the immigration office, you will spend more money getting there than just paying an agent to do it. Even disregarding the time involved. Agents will do it from around 500B, it's not a lot.
I do it in person in Chiang Mai because it's quick and I'm not too far from immigration, if I was in Bangkok or lived out in the province somewhere I would definitely use an agent (or, post it).