Apparently, $100+ codes can be used multiple times with a trick. If anyone has one that they'd want to share, that would be awesome. Of course, there is a risk if you want to use it yourself (someone could check-out/purchase before you're done putting in the code on your own itin), but I suppose you can enter it into your browser window before sharing it. There is also a "does it really work?" factor so either verify that or maybe someone who has a $100+ code they aren't planning on using would be more willing to share.
FOR CODES >$100 (one time use only) loophole:
If you have multiple windows / tabs open, you can use THE SAME CODE for each window/tab but MAKE SURE ALL OF THEM ARE ON THE PAYMENT PAGE. Then you can hit submit for each tab and the $100 or whatever denomination would be applied to EACH OF THEM.
Once the code is used, it cannot be inputted into the system. The trick is to input the code on each window/tab before USING IT. I did this last time for 4 flights. the $100 was subtracted from each itinerary.