1. A method comprising: receiving, at a first module in a point-of-sale (POS) system of a merchant, a message sent by a remote payment service system via a network, the message indicating a tip amount related to an electronic payment transaction between a consumer and the merchant, the message having been transmitted to the POS system by the remote payment service system in response to the consumer having input the tip amount into a personal communication device; and in response to the message, communicating the tip amount from the first module in the POS system to a second module in the POS system, wherein said communicating includes, without user input locally at the POS system: simulating, by the first module, a first sequence of user inputs and a second sequence of user inputs by invoking an application programming interface (API) of an input device of the POS system; transmitting, by the first module to the second module, data indicating the first sequence of user inputs, the transmitting causing the second module to enter a state appropriate for receiving the tip amount; and transmitting, by the first module to the second module, data indicating the second sequence of user inputs, the second sequence of user inputs representing the tip amount that the consumer input into the personal communication device; and updating, by the second module, data of the electronic payment transaction to reflect the tip amount.