I've bought and sold a number of cylinders on eBay and SB and at some point I got a 25 pack of 8x8x30" cardboard boxes. I have an account with UPS -- they'll give one to anyone -- that gives me a modest discount from retail rates. I drain the cylinders to 25 PSI and put a DIN port plug in place to make the valve a little more rigid. I put a folded-up towel over the valve and tape the box shut. In this particular case I jammed a pair of wetboots in so the tank wouldn't rattle around in the box. I'm shipping the two cylinders and another box with my fins, snorkel, trim weights, and dive flag.
When shipping multiple cylinders, sometimes it ends up being a little cheaper to tape the boxes together and set them up as a single shipment.
Sometimes I ship with the valve removed to prevent damage to it and to save a dollar or two since the box can be cut down a few inches. Didn't do that this time.
It works out to about $50 a cylinder, each way. Worth it? Who knows. I want to spend some quality time on the Spiegel Grove and there's no way I can do that with an AL100 of air or an AL80 of nitrox. Rainbow Reef has reasonable rates overall. They're very close to where I'm staying. I'm diving with my kids most of the time (but not on the wrecks). Had I set up all the dives with, say, Conch Republic, there would be a lot more driving and the overall charges would be higher because they do charge extra for HP100s and because of other differences in their rate structure. I'll ask Rainbow Reef for Nitrox for the wreck dives. I'll report back on how the whole thing went.
I dove the Vandenberg last year and it was something of a disappointment. There was a raging current which contributed to using more gas, and I had a AL100 at 3000 PSI for each dive (it was a double dip). Got 14 minutes on deck the first time, a little more the second. The operator asked us to ascend when we got to 1100 PSI, which I think was prudent. On the Spiegel Grove I'm hoping to have enough gas to dive my computer for a single dive and get 45 or 50 minutes on EAN32.
On the shallow reef dives, correct, it doesn't really matter much because I can get almost an hour out of an AL80 and the extra 5 minutes don't matter much.