So I had an existing customer I activated this feature on today. The customer called and asked if they could pay online this time. We typically have this feature off because we don't want the fees. If they want to pay this way we pass the fee on to them.

When I tried to send the existing invoice, there was no link in the email. Long story short, I had to duplicate the invoice and delete the previously made one and the link showed up when I emailed it. Maybe someone will have the same circumstance and find this comment helpful.

Basically, you need to create a new invoice AFTER turning this feature on.