When you buy a wowcher, you form a contract with Wowcher whereby you pay Wowcher and Wowcher provide you with a multi-purpose wowcher voucher.  As the wowcher is multi-purpose, you can choose to redeem it in a number of ways - you can redeem it: (a) for the deal that is listed in the wowcher (the headline deal); (b) towards any other deal on the Wowcher website; (c) for Wowcher Wallet credit; or (d) in the Wowcher Shop at www.wowchershop.co.uk.     
When you redeem your wowcher with a merchant, you are forming a second contract with the merchant that is separate to the contract with Wowcher.   
Under the separate contract with the merchant, the merchant agrees to provide you with the products or services in exchange for your voucher.   
Just to summarise this means you have two contracts: one with Wowcher for the wowcher itself and a second contract with the merchant for the products or services that you redeem the wowcher for.