Sorry to jump onto this thread, but this issue doesn't seem to have been resolved. I'm having the exact same problem with a client opening my invoice on a Windows machine with internet explorer. The link to download invoice doesn't show, nor does any information on how to pay the invoice. This was in a school with strict software restrictions so trying a different browser isn't an option, and I'd be really worried if you're not supporting default browsers.

I have taken your advice and downloaded the invoice myself and sent it (something I can't do from the app so it's not a good long term workaround)