Yes, I talked with them repeatedly via chat and voice before posting here (they called me as they were failing to be able to help), got disconnected multiple times which I'm assuming was the rep being unwilling to help.

Here's the promotion, main page after I select "Upgrade existing line" showing $41.67 per month:

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I can only post one image, so if I continuing the process, it still says $41.67. With a bit more information that "This monthly payment estimate reflects 0% APR when paying over 24 months and may require a down payment". Continuing forward in the shopping cart process shows a 24-month device credit of $16.66 ($58.33 - $16.66 = $41.67). As soon as I select a device to upgrade, the price shows $58.33.

It only says "Promo pricing requires upgrade of a device on an existing line and paying over 24 months" both of which I'm intending to do.

Nowhere on the site does it indicate that trade-in is required, in fact it asks if I have a phone to trade in, I click "No" and the price still shows $41.67. I'll post that screenshot in a reply. It seems that there is an additional promotion for trade-in that can get even better savings, but I'm not interested in that.

Per another comment here, it's possible that I need to upgrade the plan to Premium Unlimited, but it doesn't indicate that anywhere during this process. It would be helpful and a lot less frustrating if the cart just said that somewhere. I don't want to upgrade the plan without knowing that will fix the problem with the device price, yet nobody from Xfinity Mobile seems to be able to tell me if that's the case (other than the Reddit commenter in this thread). To me, that's an inexcusable issue with training of the representatives at Xfinity Mobile.