Tying a specific shipping method to a product

There is no direct way to tie a specific shipping option directly to a product, but there is a way to use the minimum and maximum weight fields for your shipping method to do what you need. 

The key to doing this is to create a unique weight situation for the product. Here is an example of how you might set this up for free shipping on specific products:
  1. In the pricing engine used with the product, enter a weight of zero.
  2. For the shipping method the product should use (Free Shipping, for instance), make sure that the minimum and maximum weight is set to zero.
  3. For any shipping methods that shouldn't be available for this product, enter a minimum weight higher than zero (Preferences > Shipping > "Shipping Methods" tab > [Specific Method]). 
Here is another example of how you may set this up for very heavy shipments that you use your own delivery truck (freight, etc.):
  1. In the pricing engine used for the product, make sure to enter the weight. 
  2. For the shipping method the over weight shipment should use, set the "Minimum Weight" field to the weight you would like to trigger this method in the store.
  3. For any other shipping methods you have set up, that should NOT be available when the weight gets to a certain level, set the "Maximum Weight" field just below the minimum weight you set in step 2 above. 
  4. In this situation you may also want to set a "Cost Per Unit" so that you still charge for the option and base it on the weight of the shipment.
Note - You must turn on "Will ship separately" for the product with the unique weight.  If you fail to do this, the zero weight product will be lumped together with other items in the cart and be allowed to ship via an integrated shipping method.