... " i'm curious how graduation gifts work " ...
They work just like any other gifts. People do, or don't, want to send you a gift. If they do, some will ask you what you'd like, some will ask your family or friends what you'd like, some won't ask at all but will just send you whatever they feel like.
If someone asks you, of course you can say you're hoping for money to buy a macbook, but unless they ask, you cannot say one word. Naturally you can tell your family and close friends too, so that if anyone asks them, they can say 'Oh I know Annie really wants to buy a macbook, so money would be welcome'.
HOW they send the gifts/money to them is how they would send such gifts for any occasion; the Post Office doesn't offer special rates for 'graduation gifts'.