Unfortunately, Product Upsell is not able to scale these offers in this manner. You can't allow this as a mass offer because it's difficult to properly limit the purchase quantities and you risk the possibility of customers being able to receive multiple discounted products when purchasing a single full-priced product.
To get around this limitation you could create multiple, more targeted offers.
- Buy one shirt, get one shirt free.
- Buy one necklace, get one bracelet free.
- Buy one pair of jeans, get a second pair at half price.
- Introducing "True Upsells" for Shopify
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- Why upsells AFTER checkout on Shopify will be your highest converting click