Systems and methods are disclosed for tracking user access to an advertisement and user transactions with an electronic commerce computer associated with the advertisement. In accordance with one aspect of the disclosed technology, a transaction tracking computer associates an identifier with each access (e.g., click) of an advertisement. Transactions that occur at the electronic commerce computer that are related to the advertisement will be reported to the transaction tracking server together with the identifier. In one embodiment, if the reported transaction is a duplicate of a previously reported transaction, then the reported transaction is determined to be invalid. In one embodiment, if the reported transaction occurs more than a predetermined amount of time after the time the identifier was initially generated, then the reported transaction will also be determined to be invalid.