Yes, you need to contact your buyer.  Also check that the address on the envelope matches that on Paypal to make sure the error wasn't due to you sending to the wrong address.  

You should not have to pay for another lot of postage if buyer was at fault (ie not advising change of address).  But having said that as suggested above this can cause a bad reaction from the buyer if they are so inclined.

You will need to wait to see what the buyers' response is to your message.  Make sure all messages are through eBay to avoid any issues.