Buying on trip.com and then using their confirmation to pick up the tickets a few days before would be my recommendation for you. You can pick up the tickets at any train station, it doesn’t have to be the one you’re leaving from on your tickets. This makes it easier in cities like Beijing and Shanghai where there are multiple stations. It’s gonna be a bit of a hassle no matter which way you do it but having the tickets in hand before your travel date is definitely worth it.