A Temporary Trip Permit is issued in lieu of IRP plates for travel within Mississippi. The fee for a Temporary Mississippi Trip Permit is $25 and is valid for 72 hours. Permits can be obtained through a permitting service, who will charge an additional processing fee or by calling 601.359.1717.
If you do not hold an IFTA license and are traveling in Mississippi , purchase fuel within 24 hours of entering the state and keep your receipts in the vehicle.
Source of information: http://www.mississippitrucktax.info/temporarypermits.html