I have created a free Google Shopping product feed generator for Shopify stores, and I am seeking feedback on it. You can try it at this link:
No registration is required. The way it works is that it connects to your shop (read-only) via the Shopify API, generates a tab-delimited text file at a fixed URL, and sends you the link via email. You can register the feed in your Google Merchant Center, Facebook Product Ads, or Bing Ads account, and the feed file will last for 31 days from the last refresh. You can refresh the feed once per day by simply submitting a new request via the form.
It currently supports unlimited products.
I look forward to your feedback!