Hey guys - the issue was related to the "add to cart" being displayed for the newest March Madness packs being available for sale if you weren't logged in at the time. This explains why some browsers were working (presumably the ones with the session of the logged in user) and why it was up for some.
Apologies - I obviously only tested it all with myself logged in!
Last night I finished off the upgrade of the OS, but the server's still maxing out on usage, so, today I'm upgrading the hosted hardware (by approximately 4 times). It'll be down for a few more minutes, and then we'll take a look at how it's handling the load..