Amazon requires that the products you sell on its marketplace have a GTIN (Global Trade Item Number). However, if you sell products through Printful, they won't have a GTIN. So you have to request exemption from Amazon.

If you're setting up with the Printful + Amazon integration, the final step is submitting your GTIN exemption to Amazon:

When you click the “submit” button, you'll be redirected to this page on Amazon:

Here you'll have to enter some extra info about your brand:

1. Enter that you're requesting exemption for branded products, because you sell private label products.
2. Select that you do have a brand support letter.
3. Enter the product categories you sell and a link to your website (or downloadable images of your products). Printful products fall under 4 categories:
   - Clothing & Accessories
   - Cell Phones & Accessories
   - Home & Garden
   - Sports
4. Upload your brand support letter and wait!

After you submit your request, you'll have to wait a few days for Amazon to review. They'll send you an email once it's been approved.

If you want to request GTIN exemption for new products later: 

Go to your Printful Dashboard → Stores → Click "Edit" on your Amazon store → GTIN Exemption. Then follow the steps to submit a new GTIN exemption request.