what do I do with my Justice Points once I've hit the cap?
justice points. Instead of simply tossing the bags, I went looking for ways to spend them down, so that I could get rid of the goodie bags (3). Luckily I found that Brownpaw, "what do I do with my Justice Points once I've hit the cap and I no longer need them for new PvE gear or heirlooms?" put a great comment out on WoWhead. He even listed the heirloom upgrade option that I have been playing with for my alts.
2. You can trade in old 1-80 heirlooms, along with a varying amount of JP, for a similar 1-85 heirloom. You have to decide if it's worth it for the price though -- for example, you paid the 2175 JP for the original Tattered Dreadmist Mantle, and the upgrade costs that original item plus another 870 JP just to get an additional 5 levels of experience boost... if you want to spend it, this can be done at Zulna (Horde) and Larisse Pembraux (Alliance). Note that the amount of JP required for the upgrade trade-in dropped by 60% in patch 5.3, making this a much more viable option now. In the example above, the upgrade did cost 2170 additional JP instead of the current 870!