The shipping cost is not calculated properly

Hello. I just bought your plugin yesterday.
I wanted your help, because, I turned on front end shipping, and the estimate shipping cost is being shown in the product pages, cart pages, and checkout pages. However the cost is not calculated properly based on the product quantity
The problem:
For one product, shipping cost is 3.99$, so for more than one product it should be (Quantity*3.99)$, but the plugin doesn’t not update the shipping cost properly. and shows 3.99 no matter how many quantity is selected

Please help me with this.