Yes postage can sometimes make a cheap item expensive , I would try Repco ,Autobarn or Super Cheap Auto ,if they won't sell one separately ask them if they know who will locally , try and get one slightly bigger and file the key down if they don't have the right size or try a mower shop.
The half moon woodruff key is easily made from a bolt or thick washer or flat steel ,if you have the tools (grinder and files) .
3.6mm thick is about the size of two 10 cent pieces ,too bad defacing coins is a $5000. fine, otherwise they would file down nicely.
A lot of my mower keys are about 4.6 mm thick ,that's the thickness of two 20 cent coins.
