Bait and Switch Fees

First you can't honestly believe any company would have an "official" policy to mis-lead customers to charge extra fees, that is ridiculous.  Second have you reached out to TMO via the options given to you by @tmo_lauren, I will note that T-Force on FB is very good at resolving issues.

Are you sure the $77.50 in not the total for 2 lines and tablet for a partial month?  I know my first bill was a bit higher than normal but it was because my billing cycle (and a lot of friends also on TMO) starts on the 25th of the month so my first bill had a partial month for service thru the 24th then the full month of service 25 thru the next 24th.  You also keep saying "fees", fees are like charges for activating a line, roaming, changing phone numbers etc, versus service charges so when talking to CS that might confuse the call since you seem to be applying "fees" to all charges.

FYI you actually probably did them a favor by returning the iPhone, as all carriers make very little if anything on phones and once you add in marketing, inventory costs and free financing they usually are losing money on them.

I have been with TMO for 13+ years and have never had an issue getting charged a "fee" for anything that wasn't a legitimate published fee and frankly since the "New" T-Mobile under John Legere even most of those have been eliminated.  I would suggest going online and contact T-Force via FB and have them go over your service charges and fees and make sure everyone is on the same page and understanding.  I would definitely agree you shouldn't have any charges from the 4th until you activated your SIM cards but then what ever your monthly rated dependent on your billing cycle closing date should be prorated between the 8th when you activated to your billing closing date which is usually different and not based on your activation date.