For Shopify users
When you create a customizable product via Zakeke, it gets the General shipping rates of your Shopify store relative to Printful location, even if it is a Printful product. So you have to organize your shipping rates into the Printful location of your General shipping.
You can view the shipping profiles of your store from your Shopify admin panel > Settings > Shipping and delivery.
We strongly suggest you to edit your store theme to avoid the creation of a new item in the catalog every time a product is customized as it allows you to get the proper shipping rates associated with the product.
For WooCommerce users
The Printful plugin overrides your WooCommerce shipping settings as long as the option Printful Shipping is enabled. In order to check it, go to your WordPress admin panel > WooCommerce > Settings > Shippings > Printful Shipping.
Then, you can manage the Printful shipping methods from the Printful plugin > Settings.
For BigCommerce users
BigCommerce platform is not integrated with Printful Shipping live rates, so you should choose your shipping strategy considering the shipping cost applied by Printful.