If you have enough membership that buys a t shirt, you can make good money. But, don't use the services... Use a screen printer.
What I do is this.
Come up with a design
Do a pre-sale directly to the members so you get the right size count. You can do this with paypal.
Order the from the screen printer - usually a minimum of 100 shirts to get decent pricing. More shirts=better price per shirt.
Printer ships all the shirts to you
You package up each individual order. Including postage labels from someone like stamps.com. Me, my wife, one of her friends and a few bottles of wine gets this done in an evening.
It takes a little work but you can make about $15 a shirt in profit if you do it right. So, 300 shirts=$4500 profit
If you intend to do this multiple times, there is equipment that can make it easier, like a thermal printer to print the labels, scale for postage weight, etc.