Hello,
You do not have the loginbox widget on the site, so I am unable to see existing listings or invoices, if they exist.
This is an invoicing related question: please direct your invoice related questions to the WP Invoicing forums. https://wpinvoicing.com/support/
I tried to add a listing with social importer, but it time out.
I added another listing manually. I proceed to pay, and with paypal, but after entering payment information the page eventually timed out.
It is my belief that your server is under-resourced. A 503 error is related to server resources. Please try adding more resources to try these operations again.
For further diagnosis, please supply a login that has a listing and invoice already in process. I understand you are trying to see if the error on the print button is a bug or not. As mentioned, it is probably a 503 server resource error, as I experienced 2 timeouts while using the site myself.