To ship a shirt or two, the best is USPS First Class in a poly bag. Depending on the total weight, will be from $2.66 to something over $4. The most common sizes/styles/fabrics will ship for the low $3s.
Note, you want to buy your postage online via Stamps . com or the like, or if you are selling via a platform like Etsy or eBay buy shipping through them. Else, you can use PayPal's ShipNow. All offer commercial rates, and include tracking at no cost on FC. You can ship up to 15.9 oz via First Class if you buy this way, else max is 13 oz, os some such, and prices are mocu higher.
A USPS Priority Flat Rate mailer is the best way to ship an order over 16 oz, $6.55. For hoodies and larger orders, USPS Priority Flat Rate Legal Size mailer, $6.85. You'll need to practice your folding and stuffing technique! Took me two years to realize I could fit a hoodie in one of these. The mailers themselves are free, and you can get them delivered to your house for free (only way to get the Legal sized ones). Note, PayPal ShipNow does not offer postage for the Legal sized ones.