I've just got an archive box costume card RC9. It is so much thicker that the other relic cards. Is there a thinner version? I seem to remember there being two thickness versions of some costumes many years ago with the Farscape sets.

Believe it all depends on the source material.
Looking at the checklist most of the relics look like they came from regular shirts. The picture on RC9 looks like some sort of space suit which is why it is thicker.

Didn't open any Lost in Space but for instance Star Trek Beyond had a few relic cards that were much thicker due to the source material. All the Krall cards are really thick, and the standard uniform relics are all the same thickness.