United Airlines hält eigene Refund-Regeln nicht ein

I had to pay by my credit card for Economy plus, though I had a future flight credit by United Airlines. Yesterday I have called United Airlines Germany to book the Economy plus extra. I was told that I could not use my future flight credit for better seats or Economy plus. So I finally agrred to pay by credit card.
On the United Airlines website (https://www.united.com/en/us/fly/travel/credit.html) I found: "You can use travel certificates to buy tickets for flights operated by United or United Express, United Express-marketed flights operated by another airline, and partner airlines sold on United digital channels. You can also apply travel certificates toward some non-ticket items, like Economy Plus® seating, Basic Economy seat assignments and preferred seating when booking your ticket."