Right. For that purpose, the system tells you (when you add links), on average, how many messages you will get per day. I know it is quite rudimentary for now, but I will be adding friendlier analyses tools soon, maybe even on the Pricing page - e.g., tell us what you want to monitor and we'll estimate which price plan you might want.

Also, I just got my first paid monthly subscription users this morning (1st day of launch, woot!), so I might be keeping this payment option for now.

However, I try to avoid recurring charges as much as I can myself, so great pre-paid options are a must.