We do I have egg on my face The rack isn't a seat post fitting one. Although all my comments most certainly do apply to the post clamp type...

The type of rack actually discussed is very common. It uses 4 mounting points to the frame. Two down just above the axle, usually to eyelets already in the drop outs. And two just above the rear brake on the seat stay tops, again often to pre existing eyelets. But sometimes the eyelett are not present and a "P" shaped surround clamp is required to loop around the seat stays to provide an eyelet mount.

I'm not sure what shim the OP is referring to, or how the post is involved. Andy.