I see, well it seems as they were quite reasonable about it but I would only block them if they dropped a negative on you. (feedback order always being buyer before seller whichever of them I may be). Afterall they may well come back again and at least you have a measure of their morals.
Personally I would not get into sending freebies except for regular bulk buyers and then only if it does not raise the postage cost.
Ok, if they drop a neutral I think you just have to accept it and then not leave them any feedback as a goodwill gesture as it was you fault.
Good customers are those who react well to honest slip-ups as well as those receiving what they thought they bid for, well packed and promptly delivered.