Rules to shipping by lettermail?

I am new to selling and am unclear what the rules are to shipping by Canada Post Lettermail. In particular, for items like trading cards and game manuals, can these simply be mailed in a regular envelope with some rigid paper/thin cardstock to prevent damage? Is there anything else I should know that is vastly different from small parcel shipping? Thanks for any clarification.

View Entire Topic

I hope I can attach a photo this way! Sorry if this doesn't work.  Here's a photo of Ram Board and it's weight and cost per cut 9" x 12.5" piece. It's fairly easy to cut with a heavy duty blade. It's a little thicker that 1 mm.  Comes in 50' & 100' 38" Wide rolls at Home Depot.