It’s not about the charger though. It’s about the money in logistics they are saving. By doing this they practically cut shipments cost in half: Now that’s a big money saver
They definitely did save money on shipping, but how much?
They ship iMacs and MacBooks en-masse and can manage to afford to do so with MUCH larger boxes than they are using for iPhones. Even before they cut out the charger, the iPhone boxes were tiny in comparison. They weren't going to be terribly expensive to ship and it's doubtful they really saved much more than a dollar or two a box at most (and even this is probably generous).
Environmental concerns in transportation, I suppose, do count. But even here, the environmental impact of shipping a ton of extra chargers in separate packaging is probably worse than it would have been just to include the chargers in the box. Apple cut the charger at the same time that they switched from USB-A to USB-C, which meant that a lot of people weren't going to have compatible bricks and WERE going to need to buy one.
(You could claim opportunity cost, and this is probably, I imagine, what Apple was thinking. Even this seems a bit odd because folks like myself never pay for the Apple-branded ones, many of us buy aftermarket. Furthermore, most cell phone stores don't sell Apple branded accessories, they sell their own selections. This is where a lot of folks are going to be buying their chargers anyway.)
Logistically, you're right. They definitely saved a few bucks, but I don't think it was exactly a "here, we saved $30" kind of decision. As a consumer, it just feels like nickel-and-diming at this point. They realistically probably saved a few dollars a box.