eventpage.js

var contextMenuItem ={
    "id": "spendMoney",
    "title": "spendMoney",
    "contexts": ["selection"]
};
chrome.contextMenus.create(contextMenuItem);

function isInt(value){
    return !isNaN(value)&& 
    parseInt(Number(value)) == value &&
    !isNaN(parseInt(value, 10));
}

chrome.contextMenus.onClicked.addListener(function(clickData){
    if(clickData.menuItemId == "spendMoney" && clickData.selectionText){
        if(isInt(clickData.selectionText)){
            chrome.storage.sync.get(["total", "limit"], function(budget){
                var newTotal =0;
                if(budget.total){
                    newTotal += parseInt(budget.total);
                }
                newTotal += parseInt(clickData.selectionText);
                chrome.storage.sync.set({"total": newTotal}, function(){
                    if (newTotal >= budget.limit) {
                        var notifOptions ={
                            type: 'basic',
                            iconUrl: 'budgetty48.png',
                            title: 'Budget Limit Reached!',
                            message: 'You have reached your budget Limit'
                        };
                        chrome.notifications.create("limitNotif", notifOptions); 
                    }
                })
            })
        }
    }
})

chrome.storage.onChanged.addListener(function(changes, storageName){
    chrome.browserAction.setBadgeText({"text": changes.total.newValue.toString()});
});