Just looked at the C-Store here on the web, and it is indeed confusing. It quite clearly states This item will be unlocked for every character on your account.
This suggests that every character on the account can claim a BOFF from a single purchase. What I assume they mean is that any character on the account can claim the requisition. Thus, a single character gets a single BOFF.
This is how it used to work, prior to Season 5. Has it changed?