Can you choose how much to pay monthly? Do you always have to pay? What happens if one month you don't "flattr" anything? What if you want to "flattr" one thing

Can you choose how much to pay monthly? Do you always have to pay? What happens if one month you don't "flattr" anything? What if you want to "flattr" one thing more than other things? I don't think it's very clear at all. Also, I'm not sure I'd want to pay for something every month.