I just got this stamp from Hero Arts, and I think it's going to be a favorite! I also found a cardstock where Copics will not show through. Amazing. It's Gina K's Pure Luxury. It's so heavy, so be careful not to crack it when it's folded, but it's lovely. It's smoother than PTI White, so the Copics don't absorb the same way--hence the no show-through. I was finally able to make a one-layer, colored card (so of course, I've made 5 of them).
Paper: Gina K Pure Luxury White
Ink: Memento Tuxedo Black, Copic markers
Stamps: Hero Arts Delicate Blossoms, Mint Motif Very Square Sentiments