There are four different plans:
You can change your current plan at any time.
The free plan offers a modest amount of storage and monthly traffic quotas.
The paid plans offer increased amounts of traffic and storage quotas to meet your needs.
Upgrading takes effect immediately, while downgrading happens at the next 30-day cycle of your plan.
On the free plan there are ads in the Spaces Overview and File Manager.
With a paid plan there are no ads.
For paid plans you can find invoices and receipts for each billing cycle in the Account Overview.