Instead of buying gigabytes, you rent a connection of a given speed, in megabits per second, for a number of hours or days. Everything you move inside that speed is included. The limit is real, though: it is the speed, not the amount.
This model only works where traffic is cheap to carry, which means datacenter addresses. It suits heavy, steady jobs against targets that accept them. On PiratProxies the unlimited datacenter plan exits from the United States and can be rented by the hour, the day, the week or the month.