What are the downsides of using a visa agent for a retirement visa process in Jomtien, Thailand?

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Are you asking if there is any downside to using an agent, or are you asking if there is any downside to doing so in Jomtien?

If the first one, you will be "stuck" using an agent next year and paying again. If that isn't of any concern to you, then carry on.

For the 2nd one, if you were going to use an agent anywhere, then Jomtien is probably one of the cheapest places due to the volume of visas the agents there do. But if you plan to live somewhere else in Thailand then it might be better to do it there.