Partial payments for event registrations and updates to existing price field selections

Provides back-office staff with the following additional capabilities:

1. Record partial payment for an event registration. Then record one or more additional payments until the registration fee is completely paid.

2. Modify price field selections for an existing registration (Paid or Pending pay later). For Paid registrations, record an additional payment OR a refund as needed if the new event fee is different from what was previously paid.