I need to give 20% disctount on the postage fee, but Wix only permits discounts on the product price. Can I do this with Corvid? Thanks!
Wix elements and components (like Wix Stores) are proprietary, thus the only control you have over them is what they give you.
You could create your own alternative and then you would have the ability to customize more features.
However, that means that you would have to code the flow of your checkout from scratch, which is a lot of work and requires coding knowledge.