WASHINGTON – President Donald Trump has personally pushed U.S. Postmaster General Megan Brennan to double the rate the Postal Service charges Amazon.com and other firms to ship packages, according to three people familiar with their conversations, a dramatic move that probably would cost these companies billions of dollars. Brennan has so far resisted Trump’s demand, explaining in multiple conversations occurring this year and last that these arrangements are bound by contracts and must be reviewed by a regulatory commission, the three people said.