Cancelling Phone Trade-in?

I couldn't say no to a sales person and agreed/signed to do a phone trade-in & phone plan. I regretted it only a few hours later and tried to cancel it, but the order had already been processed so it was too late.

The live chat support said that I need to receive the new phone and then return it in order to cancel. Has anybody else gone through this? If so, how was it like and were there any points to be careful about? I'm worried because the amount of money involved is pretty huge for me, and I don't want to make another mistake. 

Thank you. In the future I'll try to be stronger and say no to these salespeople - I was just trying to ask about my wifi and here I am in this conundrum. Have a nice day everybody!