Hi all. Recently purchased a used 2008 Specialized Expedition Sport. The seat post was just a few inches too short for me, so I bought this guy to replace it. The specs on the bike page say its a 27.2 mm seat post so it was an easy choice when I was deciding what size I wanted. However, when I got the seat post today it didn't fit! Then I learned that the size of the seat post is on the seat post. Apparently its 30.9 mm. I think luck might be on my side though because its a little too small, rather than a little too big. Should I just get a new post (I don't see many options at that size and would rather not return it to Amazon) or should I just get a shim? I am not quite sure how they work. Do I need to get one that goes exactly from 27.2 to 30.9 or is it more like a range of widths? For example, if I got one that was for 27.2 to 31.6, would the shim work for any value in there? Or are is it an exact thing so that the shim can be fully inserted into the post hole?

Thanks for the help!!