Upon trying out this plugin, it displays items correctly but there are two major issues that are causing it to ultimately not work:
1) The plugin inserts a "bar" across the top with two dropdowns, one for "Product Category" and one for "Name" (and a link that says "basket" all the way on the right). The Product Category & Name dropdowns do nothing when you click on them, kind of like if you clicked on an image file that has no hyperlink appended to it.
2) Under the picture of each shirt is a dropdown for size (like S, M, L, etc.) This dropdown does not work when you click on it, but strangely moves a few pixels to the left when you click it, and back to where it was when you let go of the mouse.
Please let me know how I can fix these. If it helps, I am not a premium Spreadshirt user.