I order from Big Cheese as well, sometimes it takes a little creativity to get the first couple of rats free from the tray they come in. I like to think of it like F/T Jenga, takes some wiggling around but they come free.

The tray of 15 small rats usually lasts me about 2 weeks to finish; I store them in a ziploc freezer bag. The "storage" sized bag is big enough so that I can trim a little bit of the stryrofoam off the rodent tray and then fit the whole thing in the bag.