A quiet month costs what a busy one costs. If you write nothing in February you still pay February's figure. We would rather you read that here than work it out from an invoice, and if your writing really does stop for half the year, the month-at-a-time option is there precisely so you are not paying through the silence.
What the figure is actually for is having that band available for the month. The allowance is there whether you use all of it, some of it, or none of it — which is the honest reason this is a subscription rather than a meter.
Writing more costs nothing. Charged by the message, every letter is a small argument with yourself about whether it is worth sending. That is a bad way to run a shop's relationship with its customers, and it is a worse way to run ours with you.
The list is counted, not the sends. So the one thing that will move your bill is your list growing, which is the one change you will see coming.