One way to make your paper voucher ready to be used online, is to book a refundable trip far out in the future, using your paper voucher. Then next time when you have a real trip, you can just phone in to change the refundable trip to your real trip.

I kind of like paper voucher, if not for anything else, but the advantage of extending the expiration beyond a year, as well as no counter ticketing fee if using a paper voucher from ticket credit. I still have a paper voucher which origin dates back in 2007.