I don’t understand – if the keyboard arrived with the touchpad not working it is not your responsibility to send it back – the vendor should pay. If it failed after you have has it for a while then it is a warrantee claim but again it should not leave you out of pocket
Have you tried to open the keyboard and check is the touchpad is unplugged? to open it you just need to slide a thin plastic tool from the usb port and be carefull with the cables
Have you paid it with Paypal? In my country if you pay something with paypal, they pay the retourning costs. Im not sure if that rule is valid worldwide or not. In any case, try to contact Paypal in case you have payed with it.