1. Can I offer both credit by products and credit by order at the same time?

No that's not possible.

While setting the campaign, you will be able to choose Cashback by order or Cashback by Product. However, only one of them can be activated at a time to avoid overusing.

2. Can I offer credit as a percentage?

Yes, you can set the Credit amount as a percentage of the order/products or a fixed amount. Especially, with percentage, you can set the maximum amount to avoid big credit amount.

3. Koin considers Discounted Price or Original prices? How about Shipping Fees?

Koin will consider After-Discounted subtotal. Meanwhile, it will not calculate Shipping Fee and VAT.

For example, in 2 orders below, I set the Cashback by order as 10%. Koin will take into account the discounted subtotal: $337, and the credit amount for this order is $33.7. The VAT and Shipping Fee won't be added.