Shipping CDs as invitations to Mexico.. they want to charge me tax?

It's a customs thing, if you fill out the air bill and (you may need a Commercial Invoice) completely giving a good description of what the CD is for and what's on it, you may not be charged taxes. But this is entirely dependent on what customs agent clears your letters. Customs is looking for things that can be resold on the street and they want (like every good government) to ensure that they get their share of the profits. FYI If you lie on the air bill and customs catches it, odds are they'll destroy your invitations and no one will know about your wedding.