I found that ball-point gel pins work the best....

My invitations had texture to them (when you ran your finger over the paper you could feel it), so regular Bic pins didnt work well because you could tell where the paper was raised...

Sharpies, even the really thin ones used for writing, can spread through the paper fibers (like how they do on damp napkins)

Just give them enough time to dry before stacking them on top of one another...

If you are sending your invitations during colder months, you really shouldnt have a problem. Usually the most problems are during the warm humid months (I sent my invitations in North Carolina in August - hot and humid)

Good luck!