So if I got an Alex Linnmon desk and one of the metal parts that control how the drawer slides in and out was broke off whenever I opened it from the box can I bring in the one piece to the store? Or do I have to bring everything?

IKEA has a forgiving return policy. If you want to return the whole item it's possible. But if you just want to replace that one broken piece I'm sure just emailing or calling with a proof of purchase at hand, they'll gladly send you a spare. They might even sell it on their website. Check the manual to see what the piece is called, you can also find a digital version of the manual on the IKEA webpage of the desk. So your best bet would be to first search up your desk, read the manual and find the extra piece. Then order it.

Yes

I had about the same experience as @ttalgi. Go to IKEA. You will have a great experience. If a part is missing, they will work with you to get you the part.