Being a retired USPS employee, may I suggest that you start sending the items that you sell in padded envelopes, not white letter size envelopes. The machine that cancels the letters makes some 180 degree and some 360 degree turns, therefore ripping the envelope and sometimes damaging the chips.
I receved a letter sized envelope today that was ripped, but luckily all the chips were there and not damaged. You can buy 4 by 8 padded envelopes in quanties up to 500 on ebay. Happy selling.