Once a user clicks the add-to-basket button, use the "Create a basket" resource to create a Spreadshirt basket with the selected sellable in it. Associate the returned basket id with the customer's session. You can now use the fields from the basket payload to assemble a drop-down UI component or something similar. Use the "Update a basket" resource for subsequent customer interactions.
6. (Optional) Support promotions
If you want to show promotion banners in your shop system and enable promotion discounts provided by SpreadShop, we recommend to check out the Current Promotion resource.
7. Redirect to the Spreadshirt checkout
Finally, you can redirect the customer to the Spreadshirt checkout using the "shop checkout" link from the basket response. We will take over from there.