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I want to be able to just fund a bitcoin address given to me by the registrar, and the registrar withdraw payment as necessary when it is due for renewal. Is this possible anywhere?
I don't think so, I havn't seen any.
Although most registrars have a virtual user wallet (not bitcoin though), holding funds for automatic renewals.
I guess it's an easier route, compared to the complexity and risk of managing an online bitcoin vault.
3rd party Bitcoin payment processors could tackle this, making a bitcoin address work like credit card, supporting recurring payments.