Can we add/remove shipping costs?
Can we add shipping costs if a customer redeems a post purchase promotion?
Yes. You can optionally add a flat fee shipping cost for native post purchase offers. You can also skip the flat fee shipping cost when the order already has free shipping.
Can we remove shipping costs if the post purchase promotion passes the free shipping threshold?
No. Shopify does not support any way to remove shipping costs after the order has been placed.