The default configuration is to add all the shipping prices together for each product in the cart, and where the customer has added the same item more than once the shipping price for this product is multiplied by quantity.
It is possible to change this behavior, you can use the shipping_addon and shipping_is_percent attributes installed as part of the extension to do this.
Shipping_add_on is the price or percentage you wish to charge on additional products. E.g. if you set the shipping price for Product A to be $5, and the add on price to be $1 then a customer with 3*Product A in cart will be charge $7.
Shipping_is_percent is used to say whether a percentage of the shipping price is used on subsequent items in the cart. So if set to yes and you have a shipping_add_on of 10 then a customer with 3*Product A in cart would be charged $6.
Also available is an additional qauntity surcharge feature in the configuration panel of the extension.
The switch is used to say whether a fixed amount is added or subtracted off the shipping price on subsequent items in the cart. So if set to "-4" and you have 2 items in the cart with Product A shipping set to $10 and Prodcut B shipping set to $20, the total shipping price will be $26.
Within the Product rate shipping method you can set an overall maximum shipping cost. If empty it will be ignored
It is possible to set a default flat rate shipping cost which applies if no products in the cart have a shipping price defined for them. This can be useful if you want to use flat rate shipping, but apply a surcharge on a few products only.