I configure my store to use Flat Rate for shipping. When I add an item to cart, and go to checkout page, the shipping rate is there with my amount of money I configured (Let's say it's $20).
But when I add one more item into the cart, the shipping rate still shows up, but the price now is $0. This only happens when there are more than one items in the cart, no matter whether they are the same product with the quantity of >1, or two different products.
Could anyone please help me to fix this problem? I use Magento version 2.0.7. If there is any needed information, please let me know.
Thanks in advanced.