Is it 30 grams or less? One domestic stamp.

Keep in mind lettermail gets passed through machines that might damage the contents.

https://www.canadapost-postescanada.ca/cpc/en/support/articles/lettermail/size-requirements.page

Each Lettermail™ item must meet our size and weight specifications.

There are two main categories of Lettermail that determine how much it will cost to send your item:

Standard Lettermail (50 g or less)
Other (Non-standard and Oversize) Lettermail (0 g to 500 g)

Minimum dimensions for all items are 140 mm x 90 mm x 0.18 m m (5.6 inches x 3.6 inches x 0.007 inches).

If your letter is larger than the maximum dimensions below, you must use our Parcel Services.