Im am setting up some Google Add to Cart buttons and am having trouble calculating proper shipping. My entire store has items that have product-weight classes to calculate the shipping. The shipping table is retrieved from Merchant Center.
I have one item that needs to have flat rate shipping, so it has a product-shipping-first and product-shipping class. When this item is in the cart with any other products the flat rate shipping overrides the entire cart and the customer is only charged the flat rate shipping.
I want the cart to calculate like PayPay does by default. It calculates all the by weight products, then adds the flat rate on top of that total.
Please note that submited shipping instructions in the posted shopping cart will take precedence over the Merchant Center, thus settings in the Merchant Center are ignored.
In your case, using the product-shipping class will create a "shipping-methods" tag in the shopping cart XML, thus overriding the Merchant Center shipping instructions. Try to find a way to use one or the other.
Some shipping troubleshooting hints here:
https://developers.google.com/checkout/articles/Troubleshoot_Merchant_Issues