I use Zoho Invoice for invoicing my billboard customers.  I have a few customers that want to pay using credit cards and ACH.  As a result, I have integrated an Online Payment Gateway (Stripe) for these customers.  I currently charge these customers a "Convenience Fee" of 3% for using this service as I typically only take checks as payment (and the gateway charges 2.9% + $0.30 per transaction).  I do this by creating a separate line item on the invoice and adding 3% to it.  I would like to offer this payment option to all customers "if" the online payment gateway would "automatically" charge the customer an additional 3% fee.  Is this possible?