Discount is only on refurbished items. So battery pack wouldn't count.
The first time through (at the beginning of May) I bought two 600s and got the $125 discount. Unfortunately, when I went to buy another two, they were sold out. At the time I did some quick math and figured out the difference between the $125 off for two and 15% off was something like six bucks each.
Of course, if you can find something right at that $500 sweet spot, you can score a great deal.
I find the Canon Refurb store fascinating. Usually, a day or so after the sales end, many of the items that were out of stock magically go back in stock. Then, another sale begins and suddenly things go out of stock again right away. During the sale, some items may come back in stock for an hour or so, but it is pretty random.
I'm set with 600 RTs for the time being. One of these days, I hope to score a 200 2.8 prime, but I've found that one a real challenge.
I'm not complaining, in fact I kind of enjoy the game. But, I'm pretty much know that Canon USA is manipulating the stock. I wonder if they have an office pool going – "Let's put ten 70-200 2.8 zooms on the site and everyone in the pool guesses how many minutes it will take to sell them out."