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Not anymore you can't. The NXEC website, which is still as good as it ever was for mixing single fares to get the cheapest return fare price, has - without any notification at all - ceased offering transactions using AMEX and has also removed all signs of its 11% online discount price from all its fares.
Now the website offers no advantage over others for the cost of advance purchase tickets. NXEC also removed the airline-style white tickets that its predecessor GNER used to issue for web-purchased tickets; now for an advance fare for travel between Grimsby and London King's Cross for two people you're issued with no less than TEN generic credit card-sized orange train tickets to cover the journey. The last time I used the Virgin Trains website they didn't, so perhaps they'll be my next port of call for train tickets along the East Coast Main Line! (GL)