As of two weeks ago, you can no longer add manual shipping rates using the Google and Youtube app in Shopify. In Google Merchant Center, you will get a message saying Shopify handles Shipping (Delivery) settings, but there is no button to add your shipping rates.
Solutions?
There are two ways forward.
Option 1 (untested):
Disconnect Shopify in the apps and services area. Check if the add shipping options appear. You may need to wait a few hours. Then, once added, connect the app again. You will most likely need to connect inside the Shopify app this time.
I am unable to test this as none of my clients have this issue, and I don’t want to put any downtime on them for no reason.
Option 2:
For a straightforward solution, revert to Google Merchant Center classic. Simply click on the question mark in the top right corner, then choose ‘Use classic Merchant Center ‘.
Could you please explain how I can set different shipping times and costs in Merchant Center? My products have tags that assign them to different shipping categories. How can I use these tags in Merchant Center? If that’s not possible despite manually changing the delivery times, they keep reverting to a time I did not set. How can i have it set to the time i want
Sounds like you are using a google and youtube app, if so, you will not be able to manually add your own shipping. If you want to do this, you need to change your data feed app, I recommend using: https://apps.shopify.com/multiple-google-shopping-feeds