You'll be a big fan of this since I know ya mentioned being a big fan of loopholes in sales like this haha.
Essentially, yes to what you said. If you only want the Fillmore box set and don't see any other albums around $77 to make the deal worth it. Add two other items that are as close to that price as possible and go all the way through checkout and pay for all 3. It will ring up as each item being 33.3% off if they are all the same exact price. Immediately cancel the other two items you did not want, and you will be only charged for the one item you want with a 33.3% discount. I ended up doing this a few times. There were 2 items I wanted at $125 and I also added the Police box set at $125. Ordered all 3, cancelled the Police box set and ended up paying $90 each for the other two after tax.