On supported products you can upload your file straight into the online editor and see it placed on the product before adding to basket. That preview follows your item into the basket and checkout, so you always know which design belongs to which item.
Good to know: this basket preview is a quick visual check. After you order and we run our pre-flight check, two files become available: your print proof — a clean preview of how it prints, without trim lines or print marks — and the production PDF, which does include the print marks. (How do I approve my proof?)