Here are a few ways by which you can fix the shipping related issues in Google Merchant Center.
There are a few reasons because of which your products get disapproved because of shipping reasons. It might appear as a warning "missing value shipping" or as products disapproved due to the same reason. Here are some of the ways to fix them.
1. If your merchant center shows a warning, then you need to look if you have defined shipping for the locations you are targeting. In case you have not, then you need to add shipping details.
2. If your products get disapproved, you need to see if you have added the correct format for shipping. Google Merchant Center does not approve carrier based shipping, so you have to add shipping based on price, weight, or flat rate shipping.
3. You can also see if you have copied Shipping details from Shopify to Google Shopping Feed app. You can watch the video below and follow the steps.