Unfortunately, shipping restrictions like these can’t be handled using Shopify’s built-in shipping calculation functionality and you will need a third-party shipping calculation app. Our shipping calculation app, Intuitive Shipping, can handle the type of setup you’re looking for.
Using our Conditions system, you can assign Conditions to your Shipping Scenarios (where you set up your shipping rates in the app) and create a Condition that if a hazardous item is added to the cart, it will be an extra $28/item (we strongly recommend relaying this information on your product pages and your shipping info page so customers aren’t surprised at checkout). You can also set up the app so that when a hazardous item is in the cart, USPS and expedited air are not a shipping option at checkout. If there are State/country restrictions for these items where they cannot be shipped to, you can set that up in the app, too.
We do have a built-in user guide and tooltips, but since your setup is a bit complex and has a few moving parts, I encourage you to book a call with us so we can further discuss any restrictions/needs you have for your shipping and we can guide you with how to set it up in the app. You can reach out to us at email@example.com if you want this type of assistance.
Intuitive Shipping is free to use while in Testing Mode (which it defaults to), and once you make your rates live in your store is when your 14-day free trial starts so you can take it for a test drive in your live store.
Just a heads up that all third-party shipping calculation apps require Shopify’s Carrier Calculated Rates feature. If you are an Advanced or Plus merchant, this feature is already included in your plan. If you are on the Basic or Shopify plans, you will need to get this added by contacting Shopify. The feature is $20/month a-la-carte, or free if you switch to annual billing, which also saves you 10% on your Shopify subscription (most of our users go this route).
As @SharonReeds has mentioned, shipping restrictions like hazardous materials/dangerous goods cannot be handled using Shopify Shipping. Since you are using FedEx, you can check out the FedEx Rates, Labels and Tracking app that will help you to handle dangerous goods shipping. (see image below)
Along with this, the app also helps you show live FedEx rates at checkout, print FedEx labels and also handle FedEx tracking.
Hazmat or dangerous goods shipping is something which is not directly provided by Shopify as most customers do not use this. In order to enable such special services, you need to use the Multi Carrier Shipping label app. You can tell to enable the Hazmat in your app account and this way the associated charges will be levied with shipping.