Misorder, Frustration, and Watsons' Customer Service Runaround!

I placed an order, and to my surprise, the incorrect products were delivered. The invoice for the items I bought was correct, but the products I received were completely different. When I contacted Watsons about the problem, they advised me to return the items. I suggested that they arrange for a courier to pick them up instead of going to the post office, as going to the post office is an extra expense. Why should I pay when the error isn't mine? Their customer service representatives initially advised me to return the products. They then promised to send the appropriate ones. I asked that they pay for my expenses. They have, however, resorted to communicating via email. In the email, they mentioned giving me a 50 TL coupon at first, then refusing to exchange the products and insisting that the price difference be borne by me. I don't want to pay for their error, and a 50 TL voucher only covers my travel expenses. Watsons has been causing me problems for two weeks and has yet to find a solution. Despite my innocence in this matter, they are torturing me. This is my final point of contention. Watsons is delaying their response, so I intend to escalate the situation to the appropriate authorities.