The “Pay for Invoice” link on the Invoice should not go to a 503, but the /wpi-checkout/ page.
Ideally, I guess that link should not be there on an invoice that is to be paid with a bank transfer or the sending of a cheque.
If we need to investigate why your setup goes to a 503 page, post your WP admin details in a private reply and we will have a look for you.
I will also get the developers to have a look at the question whether the “Pay for Invoice” link should be there and/or that link can be customized to go to a different URL than the /wpi-checkout/ page.
We had another similar ticket, and the issue was due to the W3 total cache plugin. We will be investigating more with this issue and meanwhile, I would recommend using another caching plugin and deactivate W3 total cache plugin.
To open a new support ticket, first select the relevant forum and then click the "New Post" button. Example: You need support for the core plugin? Click on Core Plugin support and click on the "New Post" Button.