It is possible to create a rule where you specify that it shouldn't apply for certain SKUs. See picutre below. This will give you free shipping. However, it doesn't set the shipping cost to 8 USD. As far as I know, there is no rule that will set the shipping to a certain amount else than free.
Three options I can think of:
- Use a fixed discount. Lets say your shipping is 50 USD. If the condition is true (5 +, in the category and not one of the selected SKUs), deduct fixed 50. If it is one of the 8 USD products, deduct only 42.
- Set the shipping amount to 8 USD for the complete store. The rule will not apply.
- Develop a module, and add an attribute, such as "special-shipping", use that attribute to assign a higher rate shipping.
- Get a plugin / module for Magento, such as e.g. https://amasty.com/shipping-rules.html.
