Account Credit / Scholarship Auto-Apply at Checkout

We have no credit or scholarship balance concept today — if a co-op wants to give a family a credit, there's no place to put it. Competitors let a credit balance apply automatically at checkout, skipping the card/bank form up to the credit amount instead of just sitting unused. Scope: a credit/scholarship balance per family, auto-applied against the next invoice(s) at checkout, with a running balance visible to both the family and director.