Do not pay for something that you did not receive

I ordered a shower mixer and shower head which is 2 items. the consultant from takealot admits there was a picking error. I only received 1 item They now want me to pay for the 2 items. I told them that I am only prepared to pay for the 1 item that I received. How can I pay for the other item that I did not receive. Totally unprofessional an *********