First, price. If the $135 CAD included the additive then ($135 - $16) * 0.78 = $93 USD. Your subsequent quote from another dealer is only a couple bucks difference after subtracting for the oil treatment. If you happen to be in or around Vancouver with a high cost of living and labor rates I would not consider that out of line these days by US standards for a synthetic oil change. Also, with sharply diminished vehicle sales it is reasonable to think the service department is making up some of the difference. By the way, did your receipt indicate it was a synthetic oil and what brand?
As for the rest, @Mazdioid2 may be on to something:
Maybe they use a cheap bulk motor oil and the treatment is Molybdenum to enhance it to the level of a higher quality lube, such as Mazda/Idemitsu?
Most better quality oils available have that ingredient for prices under $25US and the additive isn't necessary.
While I would not agree that most high quality oils contain moly, there is a point to be made about bulk oil. Unlike 0W20 Mazda branded oil for the normally aspirated 2.5L "with Moly" printed on the front label (while still GF5 by the way), the 5W30 Mazda oil recommended for the turbo Sig makes no mention of moly on the label. Are these oils even synthetic? The front labels don't say so. But I digress.
Back to bulk oil. Many (most?) dealers will use a bulk oil unless you specify otherwise. It is way less expensive for them and for you. Mazda is surely aware of this and permits it. Yet if one takes the manual literally, consider this: "For maintenance service, Mazda recommends Mazda Genuine Parts and Oil." Further, the manual makes no specification of synthetic oil or differing service intervals for synthetic vs conventional which raises further questions.
Next, taking the manual literally, there is no replacement interval specified for auto trans fluid, pretty common in the industry these days. Also there is no replacement interval specified for brake fluid. There's also a weird thing where the engine air filter replacement schedule for normal service is shorter (22,500 miles) vs. severe (clean or replace at 30,000 miles).
So, here's the thing. Many independent professionals and amateur DIYers alike take these manual recommendations with a grain of salt, erring on the side of caution. It's not a stretch to think dealer techs are like minded. Many would say you should not drive your car forever without changing the trans fluid just based on a color check or brake fluid based on a color or temperature check. Then you get debates over whether you should just drain and fill the trans replacing 1/3 of the fluid or have it all sucked out by a system like the BG found at many dealers. Anyway, erring on the side of caution is not a bad thing if you keep cars for a long time and are not flipping 2 or 3 year leases.
Now, the fact the dealer was lying about Mazda recommending an additive or that oil change prices are fixed was predictable. Then again, Mazda recommends only Mazda branded oil which at that price I'd presume you did not get--on the other hand non-Mazda bulk oil is not prohibited nor are oil additives per the manual.
So, that leaves a couple of possibilities. You've encountered a service department or service writer or a tech who honestly believes the additive to bulk oil is conservative erring on the side of caution and lied to you for your own good in their opinion. If it was a moly additive you'd find a number of folks here who think that's not a bad idea. Or they dumped it in just to make another $5. Or they didn't do anything at all and ripped you off. You have to judge that for yourself.
However, when you roll into a dealership at 50,000 or 60,000 miles and the dealer recommends a trans fluid service, whether it is "we" recommend or lie that "Mazda" recommends, there are a whole lot of people who agree that one or the other kind of trans fluid service at that point is a good idea. It's darn near conventional wisdom.
As far as the "go by the manual" chant, some of the people chanting would not think of driving the car until it drops, skipping trans fluid service because it passes "inspection". Many would tell you they would not use anything but a synthetic oil though there is no Mazda requirement that you do so. There's a lot of picking and choosing.
If you conclude that an additive to bulk oil is entirely unnecessary, then by all means seek out an alternative for future service.