If campus ministry, ski club or whoever are allowed to put ads in the bags, IFC and Panhellenic should be too. But as far as ads like "Go ASA", "DKE welcomes you", absolutely not. The focus should be on going Greek, not a particular group.
I don't quite get being upset over not wearing letters - how do your orientation leaders dress?
Ours usually wore business casual-type things - we certainly weren't going to wear a letter sweatshirt. Can you wear lavaliers or pins?