Assembly costs

I ordered furniture online on Saturday and forgot to check the assemble button. I called the call centre this morning and was told that you can't make amendments to my order even though it's still pending, which is ridiculous. Anyway, the agent suggested that I ask the courier to do the assembly but couldn't tell me if I pay the courier or mr price for the cost of the assembly. I asked to speak to a supervisor and , unsurprisingly, no-one was available. I asked for a supervisor to call me back but, again unsurprisingly, no-one has.