Hey there and thanks for posting. With local and state taxes not being a fixed cost, we don't have a master list you can reference of what they'll be and if/when they'll change. Sorry to hear you were surprised by the change, but tax changes are controlled by your state's government.
I had the same thing happen to me a couple of months ago. With those added FEES, taxes are like 20%. The way to avoid these fees is just buying refill cards and redeeming them on your account. You can find them with retailers online and some drug store chains.
I live in Illinois. Since last September 2017 I have purchased 3 $10 cards from a T-Mobile store and 2 from the app. I was charged 90 cents for IL tax. No other tax. It has been hard to find the T-Mobile Refill cards in either Walmart or Walgreens where I live. Walmart may sell them online, but no physical refill cards in either Rantoul, Champaign, or Urbana, IL stores. I noticed yesterday I can use my T-Mobile account to purchase a refill amount with my credit card. Well, at least yesterday I could.
Yeah I noticed this too via an online refill too.
I thought at first it was "Washington State" sales tax, since their headquarters are located there I think.
But you're answer is very thorough, thank you for posting.
Too bad about the extra charges though.