If JustOnce isn't for you, you get a full refund within 30 days of your first payment when you have used a quarter or less of your first month's allowance. Used more than that? The refund is pro-rata, net of what you saved. One guarantee per customer.
To claim it, email [email protected] from your account address with "refund" in the subject. We'll check your usage and refund to the original card.
Boost and Docs Packs can be refunded only before you start using them. An Extra Space can be cancelled at any time from Settings → Billing; billing for it stops at once.
The full wording is in section 8 of the Terms of Service.