I am always buying or making shims to level out cabinets, etc., so here is another cheap, dirt simple tip that works for me.
I removed a vertical hanging blind from a sliding patio door, and decided to save it so I could cut it down to about 3' high and see if it would work on a shop window. While cutting it, the light dawned that this stuff would make great shims. it is 3" wide, About 1/16" thick, and you can cut it with good scissors. The plastic is almost indestructible, and hard enough, yet slightly pliable. I made some into various sized shims and they work great. I also am keeping some larger chunks of it for any other uses I might think of for it.
I see them in dumpsters all the time, so the price is right.