They already did. This just came in:
Due to U.S. tariffs announced on Wednesday, April 2, PSA will be temporarily pausing the acceptance of PSA grading submissions directly to its U.S. facilities, including through PSAcard.com, from customers located in all countries outside of the United States. This is an expansion on previously announced restrictions on direct submissions coming from Canada, China, Hong Kong, and Mexico.
https://www.psacard.com/articles/articleview/15158/2025-tariff-regulations-canada-mexico-china