Hi Celessia,
Regarding Substance Share materials, they are not owned by Allegorithmic and follow a different EULA than the Substance Source one.
They are offered for free by the artists who made them, and can be used by anyone as long as the artists are credited for their work. They should stay for free on Substance Share for anyone who needs them, they are also used a lot for educational purposes!
This is why we do not offer them on Substance Source.

Regarding the extra points, it is possible to purchase some extra if you need them!
You can subscribe for an additional Substance subscription with the same account, only for one month for example, and you will get 30 additionnal downloads. You can then unsubscribe before the end of the month if you don't think you will need more points the next month, as the points do not decay over time you will keep them even if you have unsubscribed you will not have to worry about having to use them all in a short amount of time.
I hope this answered your questions!