here is what happens, every time your package passes a clerk if they notice that the postage is wrong they send it back if its in the city where you mailed it or (more likely) if its caught in the city where its going they try to collect the extra postage from the delivery point. sometimes if its small amount they just leave a envelope with the item saying please pay the postage due (the mailman pays the postage casue he/she is too lazy to go to the door for 25 cents or whatever), or they go to the door and try to collect the postage. if someone is there they can pay the extra and get the item or have it sent back, if no one is there they leave a note for them and they have to either go to the postoffice and pay the extra, or leave it in the mailbox and it will be delivered the next day. if your sending something to a friend/family dont see any harm in trying it but if your selling something or doing a trade from a matching site i wouldent recomend this, makes you look bad to your customer/trade partner.