A possibility would be a (global, publication independent) subscription service like this: you pay a fixed amount in each month (say, ten dollars). Then every time you read an article from a website that subscribes to the service, if you liked the article you click on a widget. At the end of the month, the ten dollars are redistributed to the publishers in proportion to your clicks. So payment is voluntary but the total amount per month is already allocated- it's not a matter of deciding whether to pay or not but just how to distribute the money.