A cartel is when a few organizations control something of economic value and act as a single entity in the market. For example, the 1973 oil crisis was caused when OPEC proclaimed an oil embargo. OPEC, controlling the oil market, acted as a group to greatly reduce the supply of oil to the United States (and other ally countries) for political purposes.

EDIT: In this case, payment processing is controlled by a few organizations. They have agreed to raise their prices at the same time, instead of free market style competition for the lowest price. They can do this because very few companies can do payment processing and it is very hard to become a payment processing company.