// content.js
    for (var i = 0; i < 35; i++) {
        chrome.runtime.sendMessage(
            {contentScriptQuery: "queryPrice", itemURL: skins[priceNumber]},
            (price, url) => {
                console.log(`${url}: price: ${price}`);
            });

        priceNumber = priceNumber + 1;
    }

...

// background.js
        var url = "https://pixelboii.wtf/steamvalue.json";
        var skinFullName = request.itemURL;
        fetch(url)
            .then(res => res.json())
            .then(price => {
              sendResponse(price.items_list[skinFullName].price["7_days"].average, skinFullName);
            })
            .catch(err => { throw err });
        return true;  // Will respond asynchronously.
      }