I guess I have to look at Paypal for this.
Now I am working on shipping cost, just realize I have to set the shipping cost per product.
I turn on "Use the prices per product" and input the "Price per product" = 13.04347826 (without tax). It costs $15 with tax.
I put 0 into the main prize and Tax Category = Default Tax Category.
When I tested in the check out, the shipping cost is calculated per quantity indeed but the shipping cost is not $15 but $14.95.
I put the same amount 13.04347826 back to the main price and turn off the "Use the prices per product" option, in the check out I got the shipping cost $15.
I think there is a slight different tax calculation between these two option, perhaps the decimal digits problem? Can you help me with this, I really need to use the shipping price per product option and the correct amount of shipping cost displayed in the checkout.
Also I found, when I change the quantity number in the check out and press the refresh button, the shipping cost disappeared.