In response to a conversion action associated with an online advertisement (“ad”), an advertiser associated with the ad generates a unique identifier (e.g., a pseudo random number), which is transmitted to a payment system through a user device. In some implementations, the unique identifier can be generated by a payment system in response to a request from an advertiser. The advertiser stores the unique identifier in a secure location that is accessible by the payment system. The payment system compares the received unique identifier with the stored unique identifier. If the received unique identifier matches the stored unique identifier, and if the received unique identifier was not previously generated by the advertiser, then the payment system deems the conversion action to be valid.