The Merchant API behaves slightly different as there is no defined APIKey. The account using the merchant api must first be validated using their PubAdmin credentials using the login method. This method will return a list of applications the pub admin account has access to.
The Application should present this list of Merchants back to the user to allow them to select under which merchant they are operating on. Typically the list will only contain one merchant.
The ApiKey and SecretKey for the merchant are then used for signing all other merchant api requests.
For the RedeemVoucher method requires both the OfferOptionId and the VoucherCode. To get the OfferOptionId, the application should first call GetVouchers passing the voucher code presented. It will return of list of possible vouchers. In the case the merchant has run multiple offers and there is a duplicate Voucher Code, the application should present a list to the merchant so they can select which specific voucher is being redeemed.