We did this just a couple weeks ago. My mother-in-law in California had travel credits that she wanted to apply to my wife's ticket. I made the reservation on united.com using the Book and Hold feature. My mother-in-law then went to a travel agent near her (she preferred that to driving farther to a CTO or airport), where they were able to ticket the reservation. The travel credits were applied, and my mother-in-law paid the difference.

If we had had more time, I would have tried to see if she could just apply the travel credits and then have my wife or me call to complete the transaction with our credit card. We didn't, so I don't know if that's possible.