I'm trying to renew one of the add-ons I use, but the invoice is expired. I missed it by a few days.  I click pay-now but it just tells me that the invoice is expired.  Is there nothing I can do?

Go to the product page and click to renew.

 

Go to the product page and click to renew.

 

That's how I got there in the first place.

You have to generate a new invoice. 

Or simply create a support ticket to get it done :). 

Seems to be the same issue people have with their own sites.