How add product and apply discount to that upsell product?
You can set product and custom discounts in upsell products.
Go to Funnel > Design > Products Block and add the product as you want to set in Post Purchase upsell product.
After setting the product you can see the Discount type, This discount is the default Shopify set discount which you already set in your store. So you can customise also there.
You can also able to set the extra custom discount for the Post Purchase to upsell the product. We are providing both the options Percentage discount and Fix discount so you can set it there as your requirement.