You generally can’t do this with Shopify’s standard free shipping discount settings.
In the regular free shipping discount setup, the minimum purchase amount or minimum quantity applies to all products in the cart. Shopify’s documented setup doesn’t include a native option to exclude one specific product, such as a donation item, from counting toward free shipping eligibility.
What you can do instead:
Use custom shipping discount rules, if available in your setup
Shopify documents that custom shipping discounts can use rules such as Product collection and Product SKU. This is the closest native way to make a donation product not qualify for a shipping discount.Put the donation product in its own collection
Then use that collection in the custom rule logic if your store has access to those shipping discount rules.Redesign the shipping setup using shipping profiles Shopify does document shipping profiles for offering free shipping on specific products, but not for excluding a product from qualifying for a discount threshold. So this can help in some setups, but it won’t give you a direct “ignore this item in the free shipping threshold” switch.
See Shopify Merchant Support for more details.