I did not know here that the invoice had expired. As mentioned, we need to see the issue in action at the time, and not after this has been attempted to be corrected. A user should indeed be able to renew if the invoice has expired.
This is indeed by design when you allow the purchase of only one of those items. You would renew the item you already have, not purchase a new one.
Why can't the user just delete the invoice and the renewal himself? Why can't he overcome this issue and just add the same product to the cart and cancel the previous invoice to start afresh? He just can't do any of that. He can't cancel it and start anew. He gets frustrated and either leaves or send a support ticket, abd the same story starts again.
The situation at present is that we cannot assist unless you have an item that is currently in that state that you havent been trying to correct. There have been bugs that have been corrected, but whether you are seeing one of those that has carried over, or you simply havent had the bug fix before the event, we dont currently know due to the above.