Is this service limited to walletbit users only? I.e. if my customer is using official client, can I somehow create monthly invoice for him, using your merchant services?
Bitcoin is a push technology. There is no current way for any third party service to spend from your funds in your Bitcoin.org client. (except if you were to perhaps share your private keys with that service .. which is a bad, bad idea.)
At some point I suppose there could be M of N transactions created in advance, and simply if the funds haven't already been spent the third party could process the spend when the subscription renewal time comes.
Bitcoin is like cash. So are there any subscription models that use cash? Not any that I can think of, except maybe a monthly collection at the office for the birthday gift pool or something like that.
I know the subscription commerce model for startups is all the rage now (e.g., Dollar Shave Club, Birchbox) but if you look into that, actually getting paid is a huge problem for them:
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http://blog.chargebee.com/10-common-recurring-billing-challenges-learn-how-to-overcome-them/#sthash.RZadJwfx.dpbsThe challenge for any intermediary accepting fiat and paying bitcoins comes back down to this:
For the same reason that no Bitcoin exchanges accept paypal, wepay, etc, you likely won't find any offer to send non-reversible bitcoins using funds received through these payment methods notorious for chargebacks.