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.

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I agree about  using Corplast but as well as being expensive it is fairly thick. It consumes valuable "real estate" in your package. As an alternative I use "Ram Board" which is much thinner and very tough. It's a construction product used to protect hardwood floors from damage. It has no "grain" or flutes like cardboard so it makes no difference which way you put it into the envelope. It's very strong and resitant to folds. It does add a bit of weight. I use it for all my expensive paper items that are already protected by acid free backers and plastic sleeves. Ram Board can be purchased at Home Depot in 2 different sized easy to store rolls. One size  is about $35 and the larger is about $60 but works out to be muchcheaper per square foot.  When cut into 9" x 13" sheets the approx cost per sheet is 35 cents. Be sure you get Ram Board and not the cheaper paper "sheathing" which is too flimsy.