The cash register does NOT say "NR4LIFE" or anything of that sort.. That was a joke, and you guys fell for it. There were complaints that the simpsons were left-wing, so they made that joke to rebut the claims (that they're actually right-wing).
Anyways, it's actually some number (dollar amount), though I can't remember it off the top of my head... it's the average amount needed to pay for a child for a month, or some weird statistic.