Shipping has been working great for us except for this weekend, one order got through with this:

Sub-Total: $44.99
f: $0.00
Total: $44.99

I have NO idea where the f: came from. I tried to duplicate this on the site with this customers information again, but I couldn't. Does anyone know what this could be? Obviously, I'm trying to avoid this in the future so it doesn't happen again!

Zen Cart 1.3.8a
Patch: 1::

Fast and Easy Checkout with Ajax
We have some other mod's on but this is the only time this has been affected. We added FECwithAjax about a month ago. No other problems but this one but don't want it repeated ;)