A system and methods for displaying a selected advertisement on a web page merged (injected) into a list of transactions provided by an online (Internet) banking portal of a financial institution, independently of a computer system that provides the list. A consumer's browser receives an account history page comprising a list of transactions from the portal. The list is processed locally at the consumer's machine to generate a list of currently-displayed transactions, which are then transmitted to an offer placement system. The offer placement system accesses a database of potential advertisements to identify a selected advertisement corresponding to an advertisement selection criterion. The selected advertisement is then transmitted to the consumer's machine. The consumer's machine merges (injects) the selected advertisement in the web browser with the transactions list in accordance with predetermined advertisement placement information. The injection is effected by a script such as Javascript loaded from an associated script server.