Customers receive advertisements or “impressions” related to brick and mortar merchants while accessing online content. The merchants or other entities track which impressions correlate with customers coming to a physical store and conducting a transaction (e.g., making a purchase) by comparing transaction information with information about the customer that is provided by the source of online impressions. In one implementation, the merchant creates a hash from the customer's name and account number on a payment card. This hash is compared with a hash from the impression provider that is generated using the same technique. When a match is found, it is inferred that exposure to the online impression caused the customer to make a purchase at the physical store. Merchants may pay the impression providers an advertising or referral fee based on the matches.