There are two errors that show up after the view and pay invoice link is clicked and the customer is on the pay invoice page. 

The first error message is when trying to view the invoice, "We couldn't load the invoice".

Then when trying to download the invoice. "Invoice temporarily unavailable".

I have recreated this issue as well by sending myself invoices and attempting to view and download.

I have tried various web browsers, other smart devices, clearing history and cache, etc. This has been a constant issue since we started using merchant services a couple of years ago, so I'm pretty sure it's not just a temporary issue.