var c = {
    sItem: cart.join(','),
    nPrice: sTotal,
}
User.update({ sFacebookId: sender }, {
    $push: {
        "aOrder.$.aCart": c
    }
}, { $set: { "aOrder.$.sReceiptId": sReceiptId } }, (error, result) => {
    console.log(result);
})

aOrder: [{
    sReceiptId: String,
    aCart: [{
        sItem: String,
        nPrice: Number,
        nQuantity: { type: Number, default: 1 },
    }],
    dTimestamp: {
        type: Date,
        default: Date.now
    }
}],