1. A computer-implemented method comprising: under control of one or more computer systems configured with executable instructions, receiving a first token uniquely identifying a customer and including a timestamp indicating a time when an online impression was provided to the customer, the online impression being related to a merchant; computing a second token at least partly in response to the customer conducting a transaction at a physical location of the merchant; identifying a match between the first token and the second token; and attributing the transaction conducted at the physical location of the merchant to the online impression at least partly in response to the identifying of the match.