For Shopify users
When you create a customizable product via Zakeke, it gets the General shipping rates of your Shopify store, even if it is a Printful product.
You can assign a specific shipping profile for a product from your Shopify admin panel > Settings > Shipping and delivery. Under App shipping profiles you can see the list of all shipping profiles from Printful. Click on Manage rates in shipping profile and from there you can add a product to that specific shipping profile.
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.