Measured in the coin you paid
Underpayments and overpayments are measured in the same asset as the Invoice, so exchange-rate movements do not affect them. The Invoice Detail view shows any Underpaid or Overpaid amount, with a note explaining what was done.
Underpayment
Small shortfall: treated as paid in full, and your plan is activated for the full period.
Larger shortfall, up to 15%: your plan is still activated, but its period is reduced in proportion to the shortfall (whole months plus remaining days pro rata, rounded down). We let you know about the shortfall.
More than 15% short (less than 85% received): your plan is not activated and the Invoice is marked Partially Paid. See "Partially paid invoices: settlement, deadlines and refunds".
Network and processing fees can reduce what actually arrives, so a payment that looks close to the Invoice can arrive with a larger shortfall. To get the full period, always pay the exact amount shown on the Invoice.
Overpayment
5% or less: not converted and not refunded. Amounts this small are generally used up by network fees, possible cross-network fees and administrative costs.
More than 5%: converted automatically into extra time on the same plan. Extra days = days in the Invoice period × (overpayment ÷ amount billed), rounded down.
Overpayments are never returned as funds, and extra time starts after your current period ends.
Duplicate payment
Paying the full amount more than once for the same Invoice is recorded automatically as a separate Invoice for the same plan and duration. Paying 2× gives two periods, 3× gives three, and so on; any remainder short of a full multiple is treated as an overpayment.
If a duplicate payment was made by mistake, you can request a refund of the separate Invoice it created. The refund form offers "I paid twice for the same invoice" for exactly this case.