Why can’t I use integrated shipping options for orders with custom amounts?
Integrated Shipping options are unavailable for carts with custom amounts because of the lack of weight and dimension information to properly compute for shipping fees.
If you want to make use of integrated shipping options for your orders, all items must be created products in your store. However, non-integrated shipping options like Customer Pickup and Own Shipping are still available to you.
Custom amounts via Keypad is best used for face-to-face transactions or non-cash transactions.