From the official spec (still WIP)[0]:
>For example, micropayments of a predetermined monetary value could be "streamed" to the site over time. Alternatively, the user could make one or many discreet "one-off" payment(s) of any arbitrary monetary amount, even while micropayments are simultaneously being streamed.
Also, Flattr[1] tried (and eventually failed) to establish something similar more than 10 years ago.