I have been doing the same as you, charging a bit extra on postage and then refunding. Trouble is I,m getting older and starting to suffer from grumpy old man syndrome. A couple of weeks ago I had a cocky young bug ger rub my nose in the fact he was claiming item not recieved and there was nothing I could do about it.

The stickers are a bit like a padlock on a shed. They wont stop the serious thieves, but they might discourage the opportunist.