Someone has bought an item from me and they have paid me in full. However they sent me a message to say could i send it earlier with special delivery. anyway my question is how do i now send them an invoice via ebay for the £6 special delivery thanks

You won't be able to, you will need to give them your email address (nothing else needed for paypal) to send payment direct to paypal without going through Ebay. Ebay doesn't like people doing this but if you think your buyer is legit you shouldn't have any problems. If they won't pay through paypal you will have to explain that you will send it the fastest way with what postage they have already paid. It seems a bit odd they wait until after they pay to ask you to send the package special delivery, they might be trying to get a free upgrade on postage so unless they pay before you send for the extra postage send it how you were normally.

About this matter, i think better you discuss with buyer.
and please let them understand by your reason.

Ask them to send you a cheque or postal order or to pay you through your paypal account