There are two possible reasons why an invoice's total amount would be zero:
Mews Payments fees: Invoices for Mews Payments fees are always for zero. The reason is simple. Fees are deducted from payouts, so you, as a Mews client, receive the net amounts. The final invoices are with a balance of zero because the fee has been already paid.
Automatic settlement: The invoice has already been paid because the fee has been deducted from a merchant payout.
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