I just want to point out that cardboard Flat Rate Priority envelopes are designed for paper, not other items, and are sorted on special large envelope sorting machines. When these envelopes are overstuffed or uneven, they get jammed in the sorting equipment and may be damaged. For bulky items, it is better to use the Flat Rate bubble envelope or a box. When items are excessively taped, they get stuck in the high-speed sorting equipment.
If you have an issue with a window clerk arbitrarily refusing your packaging, ask politely to speak to their supervisor or the Postmaster. When I worked as a window clerk, most customers with issues were sent to my window already wound up and screaming. I know long lines are frustrating, but it is demoralizing to be yelled at all day long. A lot of the problem are customers who show up with their mail completely unprepared. We always put them to the side and had them prepare the mail themselves.