[RFC][Draft] Checkout/Order additional fees

[krzysztofwolski]

Problem

TLDR: Add additional fees to the checkout/order.

This issue is a compilation of already reported issues and known workarounds:

• processing fee for using a particular payment method:
  • Processing fee #7526
  • Add payment price to checkout and order #10114
• additional delivery fee tax
  • Colorado Retail Delivery Fee in AvaTax https://docs.saleor.io/docs/3.x/developer/app-store/apps/taxes/tax-codes

The mentioned examples are related to different parts of the API (shipping and payments), but I see a common pattern of "Based on condition X, I would like to add an additional fee to the order". Such a use case is a common case for b2b.

Current workaround:

• Create a digital product hidden from the listings
• Add it to the checkout via webhooks
• Add custom logic in all presentation views to treat it not as "normal" product, but a fee (for example - don't allow to remove it in the cart view)

Problems with the workaround:

• need custom code for synchronization of the fees
• possible impact on reporting and stats - "fee product" has to be removed from calculating best-selling products etc.

General Assumptions

• if fees could be kept as order lines, they should be easy to filter out (unlike current solution with checking the product type, etc.)

Acceptance Criteria

• Customers should not be able to modify it via API
• There should be an API allowing apps to apply fees
• (TBD) Multiple fees can be applied
• Fee requires tax code information

To be considered, open questions

• How fees should be handled in the dashboard (refunds UI)
• Should the fee be an extension of the normal Order line, or separate array?