so current bike is 30.9 post and I need a new dropper, new frame that’s on pre order is 31.6.

Is it a bad idea in anyway to use a shim in the new frame? Only reason I can think of is not as much of the post in contact with seat tube? Don’t fancy buying 2 posts! I could always just a straight post for a month if it is a bad idea

I have used a shim on 2 steel frames and not given it a second thought, however I bought a 31.6 post for my new carbon frame as I just didn’t feel happy shimming

also worth noting some manufacturers say shimming voids frame warranty, so they must feel it makes a difference

I’ve never had any problems with 27.2 mm GravityDroppers in 30.9 seattubes (GravityDropper supplied them with your chosen shim size). Just make sure you get a good quality shim, ideally 100 mm long at least. Basically, you want the shim to be at least as long as the minimum insertion depth recommended by both the seatpost manufacturer and the frame manufacturer.

some manufacturers say shimming voids frame warranty

Giant used to supply some bikes with a 30.9 mm seattube and 27.2 mm seatpost plus shim. For the XC bikes, I think it was a weight weenie thing. Other bikes had kinked seattubes so the smaller diameter seatpost gave a bit more height adjustment.

I think the issue is some shims are not 100mm and most frames want 100mm min insertion.  Just get a long shim.
I don’t know how the manufacturer would tell then, as long as it was 100mm in.

New frame is carbon, hasn’t thought about that making a difference. Reckon I will just ride with a normal post for a month!

Shim it up, easier and cheaper than buying yet another dropper, not caused me any grief on several bikes.

I shimmed a 30.9mm dropper in a 31.6mm seat post (alloy frame) and it was absolutely fine.  The shim was plastic from Wiggle.

ran my boardman fs for 2 years with the above setup, no issues

Buy 2 shims & after careful measuring cut one short & glue/loctite it to the bottom of the post,after sliding the other one on.Also turn the shims so the split is sideways.

Bit late ordered a new dropper in the new frame size! I can deal with a straight post for a month