OP here...
Unfortunately, I can't edit the subject, but something along the lines of "trouble using certs" would have been more accurate.
I called in and the rep did a great job of straightening things out.
One of my certs was from a cancelled trip due to illness. The other cert was from a cancelled return leg that I moved to a different date. Apparently these are two different types of certificates that can't be combined.
The agent explained everything regarding the differing types of certificates and apologized he couldn't combine them. Not a problem, I understood he has a policy to follow.
He said "Which cert would you like to redeem for this trip?" and I replied "The one from the cancelled trip due to illness, because it's slightly larger ($8 difference)."
He said "Are you feeling better?" and I said "Oh yes, of course. Thank you for asking."
He said "I apologize again for the inconvenience with the website and the confusing error message. We'll go ahead and refund the $200 cancellation fee from your previous trip and apply the refund to this certificate."
I thought that was incredibly generous. When not traveling on OPM, every dollar helps!