Recurring payments with arbitrary amounts and at arbitrary times?
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We'd like to find a payment provider that lets us do something similar to Hailo, ie:

1. Users sign up and give us their credit card details/authorise us to charge their account. They only need to do this **once**.

2. In Hailo's case, users might take a cab journey at any time and be billed any amount (within reason). In our case, users might need a job done at any time, again with an invoice for an arbitrary amount.

3. So ideally we'd be able to charge users accounts at any time, for any amount, without further authorisation. This *is* possible because Hailo (and I believe Uber) have it implemented. However, I don't know if they use a third-party payment provider or rolled their own.

Something like BrainTree's recurring payments is close to what we want, but not exactly. We want to be able to bill at arbitrary times, not on a fixed schedule. 

The best option we currently have is to use recurring billing, ie save invoices and then charge them all at once at the end of the month. This isn't ideal from a cashflow perspective, though. Can we do it our way?

We're in the UK, by the way.