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 Creating 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: