People buying used shirts want a bargain. You can ship these shirts by US priority mail for about $5 I think, making them more attractive, and the post office gives you free envelopes for that.
And, I think you should start your prices lower. Although you might make less, some auctioned items end up selling for more if they start lower. And, at least 3 of your items you should have rotated the picture upright before uploading it. Shirts are more eye-catching the right way. Good luck.