With the code Atom posted, I was able to easily alter the custom interface of the transparent interface mod. I just took a screenshot of the barter menu without any mods active, put the filter bar underneath the old barter-medium.png image, saved the new image with the old name, and added the code into the interface ini file.
Since it's basically the same interface, just made transparent, it shouldn't take too long for the creators of it to add these changes as well. (Or you could change it if you're impatient like me)
If there's a mod that changes the barter menu image, you'll need to play around with the numbers in order to get the new buttons functioning correctly.