John Schneider Plugs Merchandise On the Way to Jail, Could Get More Time

John Schneider's still busy plugging books and t-shirts as he walks into jail to serve 3 days -- but he could be looking at an even longer sentence if he doesn't get busy following a court order ... TMZ has learned.

TMZ broke the story ... the 'Dukes of Hazzard' star was sentenced for failure to pony up more than $150k in back spousal support to his estranged wife, Elvira. He's starting his time behind bars Tuesday, and even recorded himself on his way to the County lockup. In one of the most Hollywood moves ever ... he encouraged fans to support him while he's in jail by buying a bunch of his gear.

Thing is ... he's got bigger fish to fry. According to court docs, Schneider has a long list of things to do before the end of 2018 and, if he fails, he faces 5 more days in L.A. County jail. Among the things he's gotta complete: file tax returns for 2014 through 2016, fork over certain financial docs, and pay Elvira half of all the money he earns.