Canadian and Mexican products were initially hard hit by 25 percent US tariffs, with a lower rate for Canadian energy.
Trump targeted both neighbors saying they did not do enough on illegal immigration and the flow of illicit drugs across borders.
But he eventually announced exemptions for goods entering his country under the USMCA, covering large swaths of products. Potash, used as fertilizer, got a lower rate as well.
The letter on Thursday came despite what had been warming relations between Trump and Carney.
The Canadian leader came to the White House on May 6 and had a cordial meeting with Trump in the Oval Office.
They met again at the G7 summit last month in Canada, where leaders pushed Trump to back away from his punishing trade war.