Yes I am only human and sent the wrong item in the first place but refunding him all his money and posting him the one he bought I though was pretty fair .

IMO, there's nothing more you can (or should) do. The buyer now has a free item - albeit through a little inconvenience in having to wait a little longer and go to the effort of having to post back the incorrect item. Personally, as a buyer, that would exceed my expectations.

All you can do now is send a message stating something along the lines that you can appreciate and understand the frustration your error caused, but having refunded all of their money, sent the correct item and a replacement for the missing item (perhaps mention that it was the only brand new one that you could replace it with, and is of comparable quality), you have done as much as you can to ensure they got the item they ordered with as little inconvenience and expense to them as possible. 

I hope you did get the original item back? 

On a side note, if it bothers them that much (and, not being privy to the message they sent, I've no idea if they were upset, or just querying the item), but perhaps they could take their full refund and buy a replacement universal of the same brand. ;\