A Dine-O-Charge credit card was one of the universal methods of making purchases, quite similar to a credit card from Earth.
Ford Prefect owned one of these cards himself, using it to make lavish purchases in the novel Mostly Harmless. In this story, he stole the Ident-i-Eeze card belonging to Zarniwoop and used it to increase the spending limit on his credit card. He then used his Dine-O-Charge to buy (among other things): a pink spaceship, all the foie gras in London, and The London Zoo, as well as also buying the hotel he was staying at for the concierge. Ford charged all of this to the Guide and InfiniDim Enterprises (formerly Megadodo Publications).
A similar thing happens in the radio series when Ford is presented with his own Dine-O-Charge card by Vann Harl. He steals Vann Harl's Ident-i-Eeze card to clear all his expenses and give his credit card unlimited buying power, then proceeding to abuse the use of it by making extravagant purchases (including giving Elvis the biggest tip he had ever received), charging them all to the Guide and its publisher.