I've dropped Media Mail and Priority Mail in collection boxes without issue. I'm pretty sure they are allowed with a PayPal label because it is not 'bearing stamps'. But if it's over 16 ounces (1 pound) you do have to go to the PO, or I suppose you could schedule a pick up for Priority packages. In practice though it will probably be delivered without issue from a collection box even if it's over 1 pound (at least in my experience.)
I've also heard anything over 13 ounces has to go to the P.O. so mixed answers there. (Got one answer from calling the USPS help line, another in an e-mail response from usps.com)
And I've heard that anything with a label (i.e. not bearing stamps) can be put in a collection box no matter how much it weighs. It just has to fit through the slot.