just e-mail Van at OTG and explain that you already have reg spacers and want to stack with camber shims for 20mm of space/camber and he would prob substitute longer bolts from stacked kit for the same deal when ya buy the cambers.
Rear spacer shims push the wheels out. Rear camber shims add negative camber to the rear wheels. I would not get rear camber shims unless your rear wheels are already very flush with the back, otherwise they will lokk even more sunken in. This is why you notice, as in this post, people stack these and use both.