You want to have your coins in your own wallet without a 3rd party service involved but at the same time, you want this other 3rd party to instantly purchase bitcoins for you when you make a payment...
How would they get this data unless you provide them with the info?
It's not like they are going to monitor some addresses on the blockchain.
Somewhere, if you wish to automate things you will have to give up either privacy or control and I wouldn't recommend either of them.
As suggested, Bittr is offering recurring purchases but they won't be able to know that at 2 am you spent 4 mbits and next day 70.
I know it was just an example but it would a really bad choice since the start of the year we only had 4 hash rate drops and with the new gear coming online probably we won't be seeing to many of those.