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What I really want is a permanent address where anyone can deposite to get the specified job done any number of times.
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Using the message signing feature of Bitcoin-Qt can be a possible solution. It works like this:
1) A customer sends x coins from address AddrC to the merchant's address AddrM;
(Now the customer needs to prove that she did send the fund)
2) The merchant generates a unique token (can be a random string) and asks the customer to sign it with the private key to AddrC;
3) The customer signs the token and the merchant verifies it;
(Now the merchant knows who sent her the fund)
4) The merchant delivers her product or services.