Add URL / hyperlink to an invoice Template (not new invoices, but the traditional templates)

I wish to include a link on each invoice. Using Invoice Templates we can add a hyperlink and even add in a small image to the invoice. But upon the recipient clicking that link, the link cannot be found.

“Sorry, but we were unable to find the page you were looking for.”

Am I missing a step? Why can a simple URL not be found (it is valid)?

We preferably want to pass through merge fields from the contact record and invoice record as URL parameters in this link (e.g. xxx?Name=~Contact.FirstName~).

The link is to enable recipients to Pay via Credit Card using a 3rd party merchant (specific to the industry we are in).

We do not wish to use the new invoices. Although they look great and simple, we actually need to display Custom Order Fields on our invoices which are not possible on the new invoices, I believe.

As we do not wish to use the new invoice functionality, and we use a 3rd party merchant not Infusionsoft/Keap Payments or a supported Merchant, we need to add a link on the Invoice Template.