Sending invoices via email in QuickBooks Online is free. Fees will vary depending on the payment method used by your customers or clients the moment they select the link.
If your customer pays directly with their credit or debit card, the transaction fees apply, which is currently 1.7% + AUD 0.20. However, if your customer paid with a credit card linked to a PayPal account, then the standard PayPal wallet transaction fees (domestic and international) apply. You can check out this link here to learn more about PayPal fees.