When you visit online stores that take Bitcoins, like
www.bitcoinstore.com, at the end of the checkout, you are presented with a one-time-use bitcoin address that is valid for 15 minutes.
What happens if I send money to that address after the link has expired (using a Bitcoin wallet like Bitcoin-QT)? Will the transaction not go through, or will my Bitcoins disappear, or will they end up at the merchant anyway?
Depends on who you are dealing with. Whoever generated the address will still have the key to it, so any money sent there will still go to them. Unless they delete the keys after you use the address once, which is a rather stupid thing to do, keys do not take up much memory, in that case the money would just be gone. So you can pretty much assume that they get any bitcoins sent to that address. If they are nice and you talk to them, they will credit your account with the money sent. If they are not so nice, they will deny getting the money or just tell you it is theirs now and you are out of luck.
Typically people give out one time addresses so that they can match payments with orders. If you send money to an old address they would not know which order the money was supposed to be paying for, so it adds work and headaches to the people you are trying to pay.
