Problem is, that t-shirts are a very competetive market. Perhaps you should try to sell more than one t-shirt per conversion. Give a discount if someone buys 3.
Or better try to create a brand with you t-shirts. Get them in very good quality, see that the printing is high quality and then try to sell for $30 or $40 each. Not sell just some t-shirts with a unique design, but create a brand instead if the designs are very good and unique. Perhaps you can put something like a recurrent theme into the designs, so that you do not just have dozens of t-shirts with random designs but a recognizable brand. Select a nice brand name - perhaps something italian - italian is always good with clothes. Call them Giovanna Tuscani or whatever name you can find that sounds good and expensive and is not in use yet. Find a name that fits the designs.