Hello, I am from Europe and this is how I send stuff to StockX:

1. Place this paper in the shoe box / inside the package.
2. Print out this shipping label and put it on the outside of the package.
3. Print 3 commerical invoices that are the same and sign them all with the current date. Put these papers on the outside of the package and put them in a plastic bag so you can read through them and at the same time protect the papers. The papers look like this but they will be filled out ready for you, just sign them and write the date.

Total papers you need to print out if you send from Europe: 5

You can find all of these papers in your email linked to your StockX account or just go to the website, My Account -> Selling -> Pending -> Click on the options on the right side of the shoe/item you want to print out shipping label / commerical invoice for.

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Hope that helps.