Yeah, the flat rate boxes (small especially) are definitely a good value when compared to the counter rates for anything going out of the same region you're in, and especially good for anything going to remote/northern communities.
Anytime someone comes in to the post office I work at with a loose item, and it'll fit in the small box, and is going further than outside the boundaries of Winnipeg (where I'm located), I just automatically tell them the Small Flat Rate box is the cheapest option, especially considering any other option would require them having to purchase an (admittedly overpriced) Canada Post branded bubble envelope or shipping box. If it's a really small item then the Regional Xpresspost Bubble Envelope is a bit cheaper, but if they want the protection of a box you can't go wrong with the Small Flat Rate.
I have also occasionally messaged sellers on eBay asking if they could ship in a Flat Rate Box if I'm interested in the product and know it will fit inside one - the Calculated Shipping rates shown on the listing often exceed the cost of the box by a large margin.
Fun(?) fact: the post office I work at was one of only a couple of pilot locations for the boxes here In Winnipeg - we sold them for a few years before they were ever rolled out nationally. When I started at the post office back in 2016 or so I think the small one was around $12.49 so you can see how much they've increased over the years.
They've always been the same size, with the three options, but there briefly existed a fourth box that was intended just for remote/northern/fly-in communities. I guess Canada Post realized they were losing too much money shipping these boxes to remote locations so they introduced this "Air Stage" box that was the same size as the Small flat rate box, but cost like $29.99 or something like that. They pulled it after like a month because it was just too confusing lol. There was a laminated card we had with a long list of postal codes that required the Air Stage box, so if someone wanted to send something in a flat rate box we first had to ask them where it was going down to the specificity of the postal code, cross-reference with the list, then tell them which box to use. And there was no way to "upgrade" a regular flat rate box to an 'Air Stage' box, so if someone bought the wrong box they were out of luck. I think we sold a single one before Canada Post cancelled the pilot and asked us to return all the Air Stage boxes lol. Shortly after that there was a price increase which I guess helped to offset some of the cost they were losing shipping to remote/northern communities. I'm sure there are still a few of these boxes floating around out there, forgotten in some companies back room or something - if one ever surfaced it would be a pretty rare item for collectors of Canada Post-related things lol.
I've never heard any rumblings of a differently shaped box - I have to imagine they've done extensive R&D to determine that the three sizes provided are most cost-effective/efficient for transport while also being of a size/shape suitable for the most amount of people.
