The number of online commercial transactions involving small amount of money is more and more increasing. For these transactions, different payment mechanisms from traditional ones are needed in order to reduce the overall costs. To answer to the growing demand for micropayment technologies, several commercial companies have recently started offering micropayment services, which offer reduced costs per transactions. In this work we present the design and the implementation of a micropayment system relying on Web service technology to conclude commercial transactions. The proposed system enables clients to access the restricted resources offered by the merchants and to pay for the received service by invoking the Web services exposed by a Payment Service Provider, acting like an online bank.