There are many different types of custom envelopes that you can order. A quick web search on “custom envelope” will turn up many different sites.
The term “custom” can be used in several ways. For some, customization may mean nothing more than a custom print job on a standard envelope stock. For others, it might mean a special die-cut envelope with an unusual shape and/or a custom window to accommodate inserts with special layouts.
As noted on our Special Die-Cut Cards page, Canada Post’s guidelines for the physical shape of pre-sorted Addressed Admail items states that an item may be rectangular, square, or any other shape as long as it will not prevent the smooth handling and processing of the item. Thus, you can use a custom-shape envelope, if you wish. Certainly you would want to get prices on this, as this type of envelope would be quite expensive.