This approach is not very straight forward and would require you to make some edits to your db and a few pages in the PowerStore. In order to make the edits in the PowerStore Admin you will need to have DataAssist. To make the changes on the other pages of the site you will need to use eCart.
The first step in getting the shipping charge setup like this is to ensure that you have a shipping column in your products table. You will need to add this in the db.
Next you will need to update both the product insert and product update pages in the admin area. The first page you should update is the insert page. On this page you will need to add a new form element to the form on the page, this will be where you enter the shipping amount. Next on this page you will need to update the DataAssist insert server behavior, on the list of columns that are being inserted into the db select your new shipping column and for the value use the lightning bolt icon to select the new form element you just added on this insert page.
The process is the same for the update page, you will also need to set the initial value of this new text field to come from the product recordset.
That is all for the admin pages, on the front end of the site you will need to update the cart object to have the new column and the products detail page. On this page you will update the add to cart server behavior, in the bindings set the ship column to the value from the recordset, this should be the same value you used on the update page in the admin area.
This will get you all setup for the shipping charge rule. For the rule you will choose total number of items > 1. Then for the calculation choose based on column subtotal and choose your ship column.
There are a few steps to this and it is kinda custom so I will not be able to guide you through the whole process but if you have any questions please let me know and I will be glad to help with whatever I can.