This is a matter I've always found irritating. UA has made upgrades electronic, which disadvantages travellers in many ways. However, discount coupons have stayed paper, which also disadvantages us!

When booking tickets outside the USA, it's really bad. Say UA offers a ticket at $1200 but travel agents discount it to $1100, which is pretty normal, there is no (sensible) way to use vouchers.

Also, the UA web site is not able to process tax-exempt travel. When connecting from an incoming international flight into the USA, the taxes on the domestic flight are waived, but UA won't do this online, even though all it requires is the international ticket number to go into the booking.

I'd love to be able to get the online booking bonus some day, but UA make it so hard.