Hi,
A solution is to not allow users to buy both type of products at the same time.
You can create a manual shipping method with a price of $2.5 and a minimum weight of 10000kg. Then, set the weight of the products shipped by your supplier to 10000kg.
When both types of products will be in the cart, the system will display an error message that you can personalize to tell the user that you can't do it.
Otherwise, you would have to customize the code of the USPS plugin to handle your supplier products. But you would have to find a 3rd party developer to do that for you.