Yes, you can - I do it often, it helps with feedback for you. I often don't know the actual price until I get to the PO to as long as they have paid with PayPal, you can go into the PayPal transaction and click the link for "refund" - make sure you only make a partial refund though, not the whole lot! I usually then send a message saying that the thing is in the post, and if there is a refund, I say "it cost less to post than I'd estimated, so I have refunded you 50p by PayPal" etc. Don't forget to allow for the cost of packaging though, and getting the stuff to the PO etc, so a gesture of refund is usually enough.