In PrestaShop 1.5, vouchers are part of a new kind of discount feature: "cart rules". They can be created and edited from the "Cart Rules" page, under the "Price rules" tab. The cart rules creation process in the next chapter, "Managing Price rules".
A credit slip is first and foremost a written proof that a product has been returned, and that the user can use it as a voucher.
A voucher is a discount code which doesn't not have to be tied to a merchandise return or a refund, and which can take many more forms than a simple credit slip:
A cart rule is basically an advanced version of a voucher: in addition to what a voucher could do in PrestaShop 1.4, the cart rules system introduced in PrestaShop 1.5 enables you to: