please help.

alright. so my mom got this promotion letter in the mail saying that she is a preferred customer, and that she can get this blackberry pearl for $49.99 after mail-in rebate, advanced device credit, and preferred customer credit. but on the back of the brochure that came along with the letter, it says, "customer must be eligible for upgrade." does that mean that her contract has to be over?

No. It means that she must have reached her upgrade eligibility date. This will depend on when she last upgraded. Or, if she has never upgraded, it will depend on when she signed a contract.

The easiest way to double-check her UED is to call customer service.