We have nearly 2800 items and Shopify claims that only 72 products are approved and 250+ are not approved in the Google channel. The biggest complaint is that shipping cost is not specified which we have no control over in the product listing or Google fields.
According to our Google merchant page only 24 products are disapproved with 99% of them approved and showing ads for.
Why is there a discrepancy between the two platforms and how can this be fixed?
Do not use Shopify to check issues with regards to Google Merchant Center. In this case Shopify is a 3rd party. Instead only use Google Merchant Center > Products > Diagnostics to check for issues.
Then whatever the issue is resolve in the relevant area.
If you have shipping issues, my recommendation, disable the automatic shipping details in the data feed app in Shopify, and manually set it in Google Merchant Center. Here is a guide: https://support.google.com/merchants/answer/6069284?hl=en
Thanks for your suggestions!