If you have uploaded designs onto Spreadshirt, you can add them to the Design Gallery. Every time a user creates a shirt with your design in the T-shirt Designer and buys it, you will be credited with your commission.
As a Shop Partner, you can also create new products with these designs and offer them in your shop. You can set a shop product commission for this product. Every time a user buys this product from this shop, you will be credited with the shop product commission.