Square Invoices Fees
You can send an unlimited number of invoices for a fee of 2.5% + VAT per transaction. Fees are applied at the time an invoice is paid by your customer with their credit or debit card, Apple Pay or Google Pay. If your customer pays with an electronic gift card, the fees will be applied when the gift card is sold, but not when used as payment for the invoice.
You can read more about how processing fees and rates are calculated.