Wikisimpsons - The Simpsons Wiki
One Christmas Eve, Cratchit gave Scrooge a Christmas card that Cratchit's son Gimpy Tom had made for him. Cratchit told Scrooge that he couldn't afford to get him a gift, so he gave him the card "to sum up our appreciation for your generosity". Although the card was crudely drawn and written, Scrooge was touched by the gesture and decided on the spot to go into the greeting card business.
Within a year, Scrooge built a greeting card empire, with cards for every occasion. His employees, however, didn't share in his success. Smarmley died of ink poisoning, and Scrooge fired Cratchit when he failed to finish a batch of Boxing Day cards on time. Ironically, Scrooge carried out the sacking via a greeting card that read "So sorry for your loss of gainful employment".
Scrooge then replaced Cratchit in the card-writing job with street urchins whom he could pay much less (thus making his business more profitable) and replace very easily, simply by "[grabbing] another from the back alley", in the event that one of them died of ink poisoning like Smarmley had. With this business model, according to Grampa, "Greeting cards became a holiday fixture: Cheap sentiment produced by the labor of child slaves!"
 Behind the Laughter