Is the transaction fee charged for every sale on my site, or just the ones that use the Kickflip app?

There is only a transaction fee charged by Kickflip when a checked out order includes a Kickflip product. Specifically, the fee applies only to the value of the Kickflip product in an order. If a customer's cart includes both a standard product and a Kickflip product, the transaction fee is calculated on the Kickflip product's value only, not the full order total.