I have read that you should not go with a can but a soft box instead. There is some requirement to heat the contents if canned, which can cause degradation of protein. This isn't so with the boxes. Someone correct me if I have this wrong.
There is another option besides those mentioned above: Labrada Lean Body RTD Shake, which is in a box.
But, if I were you, I would go with packets and a shaker. All you do is add water and shake. I think if you do the arithmetic, you'll find this to be a much more cost effective way to go. I like VPX Micellean MRP, which you can buy 20 packets to a box for $34 at dps.
Here's the scoop on protein RTDs:
Tetra packs (the cardboard cans) use Aseptic packaging. This involves the fluid contents being heated to 280-290 degrees for 5-10 seconds (note that whey proteins begin to denature around 200-210 degrees).
Canned RTDs use the Retort process which involves heating the entire filled can to 230-250 degrees for 6-10 minutes, and the high quality milk protein isolates and concentrates, including casein, used in such products as MET-Rx's RTD 40, 51, Carb Count, Lite, and Worldwide's Pure Protein shakes will not denature as whey protein substrates will.
In fact, Tetra (the manufacturer of those cardboard cans) openly admits that using this type of packaging for "protein based" drinks has represented a huge challenge and is still NOT perfected for higher protein beverages. They are still addressing huge issues as it pertains to actual protein re-hydration; the blending process of the protein solids into a liquid state. In addition, they are continually faced with transfer tube and heat exchange fouling--as caused by the protein gelling (due to the high temperatures and the protein inclusion)-- which has uncovered that using aseptic packaging for protein beverages requires special handling and cleaning of the equipment during runs (which has been shown to produce product inconsistencies).
The bottom line is that you're just not sure exactly how much useable protein you're getting in a cardboard container--stick with a canned RTD and play it safe.