Allow Admins to Apply Customer Balance to Pre-Existing Open Invoices

🔍 Problem Statement

As a Pelcro admin, I experience the inability to apply an existing customer balance to an open invoice that was generated before the balance existed, which results in inefficient manual workarounds, disrupted billing flows, and increased operational overhead.

💡 User Story

As a Pelcro admin, I want to apply an existing customer balance to an already open invoice, so that I can settle outstanding charges accurately without canceling subscriptions or recreating invoices.

🎯 Definition of Done (DoD)

A feature is done when:

✔ Given a customer with an available customer balance and an open/past_due invoice generated prior to the balance being added, the admin can apply the balance to the invoice, then the invoice reflects a partial or full payment using that balance.

✔ Given the customer balance fully covers the invoice amount, the admin can apply the balance, then the invoice is marked as paid with no additional payment required.

✔ Given the customer balance partially covers the invoice amount, the admin can apply the balance, then the remaining amount can be settled by either:

• Charging the customer’s default credit card (via Charge Customer), or

• Recording an offline payment (via Mark as Paid).

✔ This change will impact Admin UI and API, specifically:

• Admin invoice payment actions

• Customer balance application logic

• Invoice payment state transitions
  ([clarify API endpoint(s)]).

✔ This solution will include the following limitations:

• Customer balance cannot be applied to void, uncollectible, or paid invoices.

• Customer balance cannot exceed the invoice outstanding amount.

• Automatic balance application to historical invoices is not supported (manual admin action only).

• Subscriber-facing UI behavior remains unchanged.