The International Fuel Tax Agreement (IFTA) was created to make the fuel tax system easier. It also brings an end to the need for separate permits. It is an agreement among US states and Canadian provinces.
If you qualify, then it is much more convenient to register for IFTA. It offers several advantages to interstate motor carriers, including.
If you qualify for IFTA, but do not choose to register, then there is an alternative. You must get a fuel trip permit from each member state or province every time you travel into it. It is also mandatory to purchase a four-day fuel trip permit (for $30) before returning to California.