WASHINGTON — T-Mobile US will pay at least $90 million, mostly in refunds, for billing customers for cellphone text services they didn’t order, under a settlement with federal regulators. The Federal Trade Commission announced the agreement Friday with T-Mobile over billing for unauthorized charges, a practice known as “cramming.” T-Mobile, the fourth-largest U.S. cellphone company, is paying at least $67.5 million in refunds to affected customers plus $18 million in fines to the 50 states …