Offerings within the store can change. Specials are only there for a short period of time.
Every account is separate and must buy the content to have it available. You cannot use a content unlock from one account for another account. Each must buy their own. Product Keys for the content, which you may get or have auto-applied to the account when you buy from the website, can only be used once, so they cannot be used on multiple accounts. People that have multiple accounts have always had to buy things like the expansions for each account they want to have the content on. My wife and I both have been playing since pre-launch, and we have had to buy multiple copies of everything so we both could have it.
"Everyone's got one. Better to be a smart one than a dumb one." - me
"It's people like you that [I]think[/I] you know everything that give those of us that [I]do[/I] a bad name." - me
"Eat, drink, and be Merry, 'cause Pippin was a fool of a Took."