Known limitations in Customer Portal

Due to limitations in Shopify's order editing API, the following features can't be supported in Customer Portal at this time. 

Orders created by other third-party apps
Shopify currently restricts what orders an app is allowed to edit. Orders created by third-party apps cannot be edited by your customers in Customer Portal. However, your customers will still be able to cancel their orders and edit the shipping address.

Customer Portal allows editing only for orders placed in your store's default currency. Customers' orders in other currencies can't be edited in Customer Portal. 

Fulfilled items and fulfilled orders
Fulfilled items can’t be edited in Customer Portal. Your customers won’t be allowed to edit a fully fulfilled order or fulfilled items in the case of a partially fulfilled order. This is to prevent any issues on your end with your fulfillment services.

Discounts can’t be added or edited in Customer Portal. 

Line item properties
Product customizations or personalizations can be viewed on a customer's order but can't be edited with Customer Portal.