TFL/London Underground does not accept QR codes - you have to have a booking number to print tickets from one of their machines. Trip.com do not supply this. This caused us a tremendous amount of stress and anxiety and wasted time. Not being able to speak to a Trip.com rep by phone didn't help either (just recorded messages following their press1/press2 phone system). To make matters worse our return overground train ticket also didn't work. The guard told us the expiration travel date was invalid but fortunately, he took pity on us as it was late and allowed us through the barrier. Trip.com's only proposal for a resolution, via email, was for us to buy new tickets and they would look at refunding them but did not offer a guarantee to reimburse us. Beware!