I received 35mm slides from A&I with cardboard mounts back in October & November. So cardboard must be the standard material for their slide mounts. Each cardboard mounted slide was numbered but not dated. I did not use their website for other options with my slides and I don't have anymore mailers or any other info other than what you already have. I guess you may have to just wait until after the holidays for them to contact you or for you to contact them. For what it's worth, they did a great job processing my slides. Very quick.

Marc