See which payment methods does Rootlayer accept - https://rootlayer.net/gateway-fee-tax-vat-agreement/
If you already have "Credit Balance", You can pay invoices using it without Paying The Gateway Fee again. :)
Don't you have Credit Balance Yet? You can add "Credit Balance" Using our "Add Fund" Feature with below link -
https://rootlayer.net/billing/clientarea.php?action=addfunds
We Have a Frequently Asked Question as well for this below -