Contact staff and ask for a refund if you don't want it.
You can do a custom header without the CSS upgrade on most themes; on the Design page, you can filter your search just for themes with customizable headers. You have to design the header off WP.com, like in Photoshop or Paint, so to do that one with three images, you just have to stitch them together and THEN upload them via the Custom Header uploader.
Do NOT buy the CSS upgrade unless you already know how to use CSS. And yes, preview is great.
The CSS upgrade won't prevent you from having to do the graphic manipulation I mentioned above; you'll still have to do it, but doing custom headers is HARDER after purchasing the upgrade.