What I typically do is reserve a flight online, and then walk into my local airport and talk to the desk agent onsite and have them apply the vouchers on the held reservation, and get the balance on a new paper voucher and walk out. They've treated me really well with it, and they know how to take off the various taxes and fees. If you're not near an airport, then you can do the same process, just call after the hold and explain you'll pay with vouchers and they'll extend the hold so you can mail it in.