This may strike some as odd, but I found the perfect solution to my poly-bag problems...garbage bags.
I came across them 2 holiday seasons ago when I went to my trusty ebay supplier and he was back ordered on my usual 10x13 polybags. I needed a quick solution so I dug up my old bar sealer and went to Home Depot and bought some 3mil contractor grade black bags [it's the same stuff that Uline sells in a 12in roll and calls "tubing", for heat sealing]. They worked like a charm. Best thing is that I'm not restricted by the size of the order. I normally had to keep 3 sizes of envelopes and 2 sizes of boxes on hand to ship out my shirts. But now, I only keep the roll of bags and the deep flat rate priority box. Plus, they work out to about 6 cents per envelope cheaper than the poly bags. Win-win, I thought.