To further clarify some of what Subsoap said, Coil is currently the only payment sending service. Coil will send at a reduced rate once the user has paid 4.50USD but this lower rate isn't documented. It will continue paying out even after the user has paid 5.00USD (the user won't get charged extra).
There is no way for the content creator to request an amount. This is by design. It's not meant to feel transactional like a purchase. Think of it as an extra benefit of their existing Coil subscription to patronize their favorite creators and get special content unlocks.
Since you are receiving transaction amounts every second, you could lock content unless it's paying at a certain rate. This is a tad scummy because they are still paying something, and they can't "refund" the payment even if Coil wanted to.