I just called to upfare to Z on a flight, intending to use two e-certs to cover some of the difference in fare.

The agent said "Is the amount of the certificate equal to or greater than the total cost of the difference in fare? I cannot go in behind the e-cert to add in a credit card if it is not."

Can anyone confirm whether this indeed is the case when using an e-cert to upfare?

Moderator, please shift to another thread if you think this is more appropriate in the upfare discussion. Since it was related to e-cert experiences, I thought posting here was better.