Harness is compatible with Shopify! Follow these steps to "Harness your Website" on your Shopify website.
Step 1: Log in to your Harness Admin Dashboard
Step 2: In your "Settings" - click on the "Website Code" tab (or Click Here)
Step 3: Copy the "Harness your Website" Code
Step 4: Log in to your Shopify Account
Step 5: Select Online Store > Themes
Step 6: Locate your active theme and select the 3 dots (...) > Edit Code
Step 7: Select the theme.liquid file
Step 8: Paste your Harness donate button code just before the closing </body> tag
Step 9: Save your changes
Questions? See Shopify's guide to adding custom code to a Shopify theme.