I had an IWC 3717, which needed a 21mm strap too. I tried 22mm but didnt like the squeezed look. So I'd say go for 21mm.
21mm leather is easier to find than 21mm rubber though...
Long story short. You want to pay with CC over a secured connection. Makes perfect sense to me...
But what if I told you that the Noob factory is out of titanium cases? So it doesn't matter where you order and how you pay, you'll get the very same message from every TD. "Hello Sir, Sorry watch is no stock. You want other model?"
Gen Big Pilot straps are even more expensive. I'd go for Peter Watchacc or Orloff (he's a member here).
http://s831.photobucket.com/user/orloff1812/library/ORLOFF%20STRAPS/IWC%20STRAPS?sort=3&page=1