Great question — answering as the UphillTech team:
None, Seat posts have to be the right size to useable and seem to come in 0.1mm increments. Fortunately, 28.6 is one of the more common sizes. While not ideal, if you find a suitable seat post of a smaller diameter, you can use a shim to make it fit the bike. Shims for common sizes are manufactured, - e.g. 27.2 - 28.6. Last resort is a home-grown shim - people have been known to cut up soda cans. I do not recommend using shims, buy the right sized post. You should be able to find a 28.6 post. You need to make sure offset is right as well as diameter. While posts can be cut down if needed, best to buy one the correct length if you can find it.
Full answer by mattnz: https://bicycles.stackexchange.com/a/97010 (CC BY-SA 4.0).
Need it done for your business? UphillTech.
