Can Usps supervisor request resignation? Can I resign and work other station? Any contact info?

Have missed few saturdays as RCA. Sup yelled and requested or demanded I come in and sign resignation tmw. Kind of scared to go in, he got in altercation with letter carrier few weeks ago. Just started yelling at me and saying "I dont need you. You better come in Monday morning and sign a resignation letter. You better sign it." I did not argue or raise my voice. Just said I would be there on Monday. Now, I am a little timid about going. I am not apart of any union. Who should I talk to. I dont care to work at that station anymore or care to speak with him or anyone there. Want to know the best course of action to take. Is it easier to resign? Quit? These are guys that came in and replaced our PM and supervisor. Will I be able to work for the USPS again? Any info will be appreciated. Thanks