Volume-based pricing starts at a higher level. Let's say it starts at $100 to begin with, goes to $50 next and then goes to $10 as a third step. Just take that as an example. As you start your volumes, they would be priced at $100, and then when you get past a certain level, it goes to $50. When you get past that level, it goes to $10. Smaller players would be paying $100 or maybe $50 if they're medium-sized.
Flat-rate pricing would have a starting point that is much lower. Let's say that the starting point was $15, as an example. Your first transaction would be $15, and your second and last transactions would be $15. However many you go, they will always be $15.