When you set the listing up did you include a Collection In Person option on the advert? If not then if the buyer has already paid I would suggest that you either issue a partial refund, thus refunding the postage costs to the buyer before he/she comes to collect the item, or give him/her the postage costs back in cash once he/she turns up on the day.
Going forwards if you are going to be setting up any more listings and don't mind the buyer collecting in person then make sure that you include a Collection In Person option on your listings so that the buyer can select this option when going through eBay Checkout. However, it is worth bearing in mind that eBay have recently changed the rules regarding Collection In Person items, in that they have now revoked the right for the seller to offer the buyer the option to pay in cash when collecting the item. This means that buyers now have no option but to make payment via eBay Checkout prior to contacting the seller post-sale to arrange a mutually convenient date and time to collect the item.
