USPS Deliveries????

I know this has nothing to do with periods, but i was wondering if any of you guys knew what age you have to be to sign for a package? I ordered a macbook air (used my own money and i'm 18) and i got an alert thing saying that since it's over $250 a signature is required at delivery. I read somewhere that it's 21 but i don't know, 18 is a legal adult and it especially wouldn't make sense since i'm the one who paid for it, not a parent or anything. I live in florida if that helps!