Monthly transaction volume
The largest input. Volume determines how much categorisation and reconciliation work exists, so volume bands exist so pricing stays proportionate rather than punitive at the edges. A business at the low end of a band does not subsidise one at the top, and a business that grows into the next band hears about it before the change takes effect, not after.
Estimate honestly. Understating volume to reach a lower tier produces a backlog, and a backlog costs more to clear than the tier would have cost.