As source is a source. They way people need to take to see the actual source will vary and is not really relevant for the value of the source or the way you cite it.
A paid website that offers access to the source is only the intermediary. If you link to the provider, it is your choice/preferred way to get to the source. In general I would prefer to give free routes to the sources, but if the easiest is paid (or requires registration) that is the way it is.
In giving a reference to the source you should make sure to mention the real source, and not just the intermediary. So Familysearch is not the source, but the record is the source. So Amazon is not the source, but the provider of the image of the book and a way to buy it. So OpenArch is not the source, but the birth certificate stored at an official archive is the source. So FindaGrave is not the source, the headstone is the source (Unless you trust the familytree shown there, then that is a secondary source).