I am from the USA in Phuket.

 

This year I have a work permit with about 40,000 THB income on it.

 

I would like to use that work permit as proof for my finances, for the marriage visa renewal.

 

If using this route, what financial / work-related documents will I need to show immigration?

Tax payments to prove your income of 40k baht or more. Work permit to prove you are working legally.

Some offices want a copy of the company registration and etc.

5 hours ago, ubonjoe said:

Tax payments to prove your income of 40k baht or more. Work permit to prove you are working legally.

Some offices want a copy of the company registration and etc.

etc,etc,etc! looked at this myself and if you can the 400k is way easier

You need so submit withholding income tax return (P.N.D.1) for three months. 

If you have been working last year you also need annual personal income tax return (P.N.D 91)

Don't bother.  A work permit includes a residence permit so no need for both.

51 minutes ago, Cranky said:

Don't bother.  A work permit includes a residence permit so no need for both.

@Cranky, can you explain more about what you mean or point me in to the right direction regarding that?

9 hours ago, Cranky said:

Don't bother.  A work permit includes a residence permit so no need for both.

Completely false and misleading info.

A work permit does not allow to stay in the country. It only allows you to work.

You have to have a separate visa or extension of stay to stay in the country.

@Cranky why does it seems you tried to give advice about something you seem to have no understanding about?