## Roadmap Task

 👉 https://getlago.canny.io/feature-requests/p/allow-add-ons-to-be-added-to-subscription-invoices

 👉 https://getlago.canny.io/feature-requests/p/define-quantities-for-plan-charges

 ## Context

 What is the current situation?

 **Option 1:** User has to create a one off invoice alongside the
 subscription, it will create 2 different invoices.

 **Option 2:** User can add a recurring billable metric and use event to
 have this fee invoice on subscription renewal, but it won’t appear on
 the first billing subscription.

 What problem are we trying to solve?

 At subscription creation or afterward, there is no clear way to invoice
 a fixed fee that is not tied to events, aside from the subscription fee
 itself. This fee could be either a one-time charge or a recurring one.

 ## Description

 Add `FixedCharges::CreateService` to handle fixed charge creation:

 * accepts add_on id and code
 * apply default properties based on fixed charge's charge model
 * filter properties based on the charge model
 * apply taxes to the fixed charge