Hi Frans,
For Professionals and Small Businesses (P1) plan users in Office 365, it is not available to purchase additional storage quotas for SharePoint Online. Each P1 subscription allocates 10 GB initial storage plus 500 MB for each user. Therefore, if P1 plan users require more storage, they need to add users.
However, to purchase additional storage quotas is available for Enterprises (E) plan users. They can be at a cost per GB per month.
If you have further concerns about this, Office 365 for professionals and small businesses Service Description could provide detailed information.
Thanks,
Maggie Li
