According to Canada Post, if it can fit through the 2cm mail slot many items can be sent via lettermail. They do have a somewhat interesting list of items that are considered "unacceptable" as lettermail: https://www.canadapost.ca/tools/pg/manual/PGletterml-e.asp#1392028 There are some interesting items on that list... I would wager that officially they wouldn't want LEGO sent via lettermail but I've sent a number of polys via regular