A fact of life in the current world of high end road bikes is that many of the coolest bikes come with the most obscure seatpost size requirements. Good luck in trying to find a nice aftermarket 28.0mm seatpost for your Colnago C50 HP, or a 31.0mm post for a Pinarello Prince. Most high-zoot frames give you no latitude in seatpost choice. Your only choice is the stock post -- an especially worrisome issue for folks who require a non-setback seatpost such as a Thomson, but are stuck with a seatpost with tons of setback.
USE comes to the rescue with their seatpost shims. They allow you to use any 27.2mm seatpost (and every desirable seatpost in existence is made in 27.2mm) in a frame that requires a bigger diameter post. Each size shim is machined to a very high tolerance from 6082 T6 Aluminum, while the largest sizes are made from a special filled engineering Thermoplastic (UPVC) to keep the weight down. Please choose the size shim that matches the exact seatpost diameter of your frame. In other words, a "32.4mm" shim will shim your seat tube down from 32.4mm to 27.2mm.