Partial crypto payments: make the state machine explicit

Underpays are not rounding errors when gas and exchange minimums exist. Decide:

• Reject until full amount (status stays open / underpaid)
• Accept partial and close with a remainder invoice
• Auto-expire the remainder

Document it on the pay page. Unique addresses make partials attributable. Pooled addresses make partials a nightmare.

Settlematic: invoice-scoped matching so a partial is this invoice’s underpay, not “someone sent 40% of something.” Canonical: partial payments crypto invoicing. settlematic.com