A dispensing system includes a container containing a flowable base formulation to be dispensed, an additive mixing device, and an actuable pump engine which draws the flowable base formulation from the container and pumps it through the mixing device. The additive mixing device includes a body with an internal cavity, an additive ingredient within the cavity, and a flow path/mixing chamber between an input and an output. With each pump of the device the additive ingredient is introduced into, and mixed with, a flow of the base formulation traveling through the mixing device. Multiple additive mixing devices may be interchangeable for different formulations, and the mixing devices may be refillable.