How do I get a postal discount? Women on ebay are ruthless and threaten negative feedback and my rate is real?

I ship clothes which sometimes are heavy such as sweaters and jeans. A flat rate postal box costs 10.35. Some sellers are selling items for 5 dollars shipping and some ebay members threaten me with bad feedback for price gouging when i can verify its costing me almost 11 dollars to ship these items out priority mail.