This is Moira from the Social Care team at Shopify. Thanks for posting! I understand you are on the hunt for an app that can calculate a discount based on the weight of items. I'd be more than happy to help you out with this today.
I have done some digging on the Shopify app store and found two fantastic apps for you to choose from below. The first app offers a custom weight-based discount solution whereas the second app offers discounts based on tags. You have the option to tag certain products and create a discount if those particular tags are added to a customer's cart.
If none of these suggestions work out, you can let us know and we'll be glad to see if we can find a better fit for you or you can go through the apps at our App Storehere.
TIP:As you continue to customize and further develop your store, have you looked into ways you can increase your store’s visitors, and increase conversions (that is turning store visitors into buyers)? Take a look atthis blog postfor some insights on how you can do just that, starting with your product pages! I’m sure this will prove invaluable for you as a merchant here in Shopify.
If you have any further questions please feel free to thread those below.
Moira | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog