To achieve this, you will have to create Shipping Rules within Shiptheory to identify when a Shipping Profile is being used with an order from Shopify.
Setting up a Shipping Profile
Within Shopify, navigate to Settings > Shipping and Delivery > Shipping. It is in here that you are able to Create new Profile
From there, you will want to give the profile a name, for example "Free Delivery". Then, add products that fall into this category and choose where you would be shipping from and where to, add a condition and price and Rate name for this profile.
To see more regarding Shopify Shipping Profiles, click here
Recognizing a shipping profile within Shiptheory
Once you have created your Shipping Profile(s), head over to Shiptheory and navigate to Shipping Rules. You will want to make a rule using a formula for "Shipping Method". When adding a formula, select Shipping Method > Equals > Custom > (Rate Name). Make sure that you input the rate name to match that of Shopify, as this is very text sensitive and needs to be an exact match. From here, create the rest of the rule as normal for these specific orders.
Once this is done, orders that come into Shiptheory using a Shopify Shipping Profile will be recognized by the Shipping Rules by using the Rate Name of the Shipping Profile, and will match the order to this rule.
Need any help with this? One of our Support Team would be happy to give you a hand!