I am forced to rely on USPS for a lot of things (not just my aquarium stuff) and sometimes it tracks correctly, sometimes it doesn't. Priority is usually half way on targert, first class you can forget it on tracking.
USPS service has deteriorated considerably in the past year and half.
We mail order everything here, ice chests duct taped shut, live chickens, even frozen food and tires for cars and skis.
We used to be able to get anything up here from the west coast in three days. Now priority mail from Washington state can be as long as book rate from Rhode Island.