-Sign into Merchant Center
-Click Settings from the left side-bar
-Click + Shipping Method
-From here, create different conditions. You would do a carrier calculated rate for anything below 150 pounds. Then, create more conditions for the next tier of weight, such as 150-300 pounds, 301-450 pounds, etc.
Please keep in mind that whatever the single/flat rate you use is, to use the most expensive shipping price for that weight range.
Brian Roizen is the Cofounder and Chief Architect of Feedonomics, a full-service feed optimization platform that optimizes product data for hundreds of channels. He has been featured on numerous podcasts and eCommerce webinars, and regularly contributes to Search Engine Land and other industry-leading blogs. Brian graduated summa cum laude from UCLA with both a Bachelor’s and Master’s degree in Mechanical Engineering.