I once successfully got them to go back in and make the choice for me. The agent had to escalate and it took some time but they did get it done.

I was honest and simply indicated I had been traveling (which they could see from itineraries) and forgot to complete the transaction. The agent actually said, "thank you for not lying and saying you made the choice when you didn't, so I'll try to help." I kind of appreciated that, but probably because they did me the solid and made it happen.