Customs were located near the shuttle bays on JMC spaceships to process passengers commuting from planet to ship for planet leave. This process is to prevent ship crew from smuggling contraband, illegal, infectious, or dangerous goods from planet to ship or vise versa. Crewmembers are allowed a certain amount of duty free.
After Dave Lister broke up with Kristine Kochanski he took planetary leave on Mimas to get his head straight of the break up, on his way back he smuggled a cat through Red Dwarf customs. In one dimension, Lister was caught by Kochanski in the nearby locker room. ("Ouroboros", Series VII)
In Infinity Welcomes Careful Drivers it is explained that he did so deliberately and knowing this would occur and he would be put in stasis, thus allowing him to travel back to Earth without experiencing the passage of time. In the television series, Lister got the cat because he was lonely after Kochanski dumped him.