They already have your name/addy/email/cc number/shoe size etc. When (not IF, but WHEN!) they find their error, they're gonna bill you for it. Even thought it is their error, they will bill you for it. Why? Because they can and mostly because they have your info. The RAM was sent via UPS/FEDEX...ie. you had to sign for it, right? In legal terms, you knowingly committed fraud. Hey, I'm not playing "bad cop". I like free stuff and getting over as much as the next guy. I'm just not stupid.
Here's what I would do. Keep calling until you talk to someone. Arrange for them to pay 100% of the shipping costs. If they want it back badly enough, they'll gladly send you a prepaid FEDEX envelope. Therefore, you do the right thing. If they hem and haw, tell them "well, I tried..thanks for the free stuff" and hang up.