Have multiple entry, Non Imm B from U.S. Doesn't expire for months, but 90 day stamp expires 3 days before due to fly out. Don't want to overstay. Do they extend the stamp at local immigration office?

Thanks.

With a multiple entry, you have to leave the country every 90 days. On show of the airline ticket out immigration might give you a 7 day extension at a cost for 1,900 baht.

If you choose the 1,900 Baht, 7 day extension route, make certain that immigration does not cancel your visa in the process. Ask first to make certain. Strange unexpected things can happen at immigration when your not paying attention.