I don't like the shim option. I picked up a new Thompson Elite post off e-bay a while back. Buy the right size post for the frame you want to ride....don't kluge it with a shim. There is always demand for reselling parts on e-bay...likely about the same price you buy the post for...so wouldn't sweat taking a bath on a seat post. The Thompson posts are terrific and go for half if not less than half of retail on e-bay...even new posts. There should be resale on e-bay for all sizes. If you are into building bikes...be fluid with using e-bay for both buying and selling. If you know what you are doing and buy right...you can enjoy the hobby of cycling for relatively low cost.
George