You don't need agents to do a border bounce to Myanmar.
You can take longtail boats between Ranong and Kawthaung, taking 25 minutes and costing 100 Bhat. In Ranong the immigration office can be found at Saaphan Pla Pier. In Kawthaung, it can be found at the main jetty in the centre of town where the boat drops you off. Both offices are open from 7am to 5pm
If the OP is looking at doing a tourist visa extension in Jomtien, he may as well go and get it over with. 99.99% they will pick it up on extension day anyway and extend your day. Other visa extensions, sometimes they get you, sometimes not.