We have free shipping set for orders over $50. Is there a way to reflect that in the Google feed?
No, not in the feed. You can only specify free shipping on a per-product basis in the feed. You set up shipping parameters in your Google Merchant Center account. Under Settings -> Tax and Shipping. If you use their 'Custom table' based on 'Price' you should be able to set the last row so that $50 and up gets free shipping. Note that Google Shopping is only going to calculate the shipping on a per-product basis, it doesn't aggregate them like the shopping cart does, so unless you have products that are over $50 themselves, you will never see them have free shipping in Google Shopping.
-Loren