So I took a couple of hours and did just that. I managed to get it working in Chrome and Firefox. (For reference, plugin development is MUCH easier and more clearly documented for Chrome.) Here's a screenshot of what it looks like in place:
The plugin (and installation instructions) are here. I was very nervous about having others try it out, since I could only test it with my own data, and there could easily be other types of sales that would break my code. I offered it to the Synergy group though, and so far a couple of people have tried it out and said it works. Exciting!