I have one $10 unlimited tablet line on my TMO Business account. I already pay $1.60/month in taxes. Now it'll be $2.10. That's more than 20% of the bill.
I also get a 404. T-Mobile's web wizards at least catch the error rather than just mysteriously breaking things as they seem to do at AT&T.Also the link provided in the letter they sent me leads to a 404.