@Robert ******
, this worked like a charm. The only problem was that the agent at Chanwatana didn't want to extend my visa (after being "only" 20 days in Thailand, because she said I had just entered the country), so I had to convince her that I was leaving Thailand to travel around Asia and would come back after expiration so I needed to buy a re-entry permit. So she gave me the extension, then I bought the multiple re-entry permit in Dom Mueang and had multiple entries until the 90 day limit. Thanks. :)