Etihad airways extra baggage

I bought a tour through a travel agent, round-trip tickets only with hand luggage (economy basic fare). I thought that I would be able to buy a discounted luggage seat online, as it is written on the website. However, when I open my booking, there is no "add luggage" option, and ticket changes are only possible through a travel agent. The travel agent says he can't do anything. Has anyone been in a similar situation? I will be grateful for the advice.