If you have a flat rate carrier created already. but you only want to offer this when certain products are in the cart then you will create a shipping group for those specific products and then a carrier rule to manage when to hide/show that carrier. The steps below will outline how to do accomplish this.
So you would create a shipping group (for your specific products) and carrier rule for this scenario where the following would be set up in ShipperHQ:
1. Create a shipping group and name it “FLAT” for example, then click save.
2. Assign this FLAT shipping group to your products in BigCommerce as described in the article below.
3. Create a carrier rule and name it whatever you would like. (Hide Flat Rate when FLAT not in cart).
4. On this carrier rule, in the “Applicable Shipping methods” you would highlight your flat rate method.
5. In the Conditions section, you would select your FLAT shipping group in the “Excludes these Shipping groups” section seen below.
6. In the actions section, make sure you have it set to “hide” shipping methods.
So after setting this up you may realize that putting a shipping group in the “Excludes these shipping groups” section means this rule won’t apply when that shipping group is in the cart. Also, in this scenario, we had to switch our thinking to “when do we want to hide this Flat rate carrier” as opposed to “when do you want this Flat rate to show.” Often times if you think of your carrier rules this way you will be able to achieve more with less.