ember: pushPayload error in instance initializer

I'm trying to preload my cart model on init and am getting the error

TypeError: cart.pushPayload is not a function

I know that not all serializers have this function, but the one I am using (RESTSerializer) does. My serializer for application.js is:

import DS from 'ember-data';
export default DS.RESTSerializer.extend({});

The instance-initializer for cart.js is:

import Ember from 'ember';

export function initialize(appInstance) {
    let CartService = appInstance.factoryFor('service:cart');

    let payload = {
        "cart": [{
            "id": "S5339",
            "quantity": 1,
            "itemPrice": 129.95
        }, {
            "id": "BLK1642",
            "quantity": 1,
            "itemPrice": 55
        }, {
            "id": "TDS9004",
            "quantity": 5,
            "itemPrice": 99.95
        }]
    };
    let cart = CartService.create();
    cart.pushPayload(payload);

    appInstance.register('cart:main', cart, { instantiate: false });
    appInstance.inject('controller', 'cart', 'cart:main');
    appInstance.inject('component', 'cart', 'cart:main');
}

export default {
    name: 'cart',
    initialize
};

For now I am using static data but if/when I get this working I will be doing a network request using ember-network.

But I'm going down the rabbit hole on why I can't get pushPayload to work. Any help appreciated!