Will flat shipping be automatically replacing other offered rates in my Shopify store?

No, Grandville Printing can't disable any rates that are configured in Shopify, we can just add our own rates to the list.

To avoid offering a shipping option that we do not offer, you should disable any other shipping rates in Shopify, except Grandville Printing's.

Note that if your store has any products that are not fulfilled by Grandville Printing, you should leave some other shipping options in Shopify since Grandville Printing rates won't be returned for such items.