Hello Gotoashou22
Since the Glacier Express is operated by the Rhaetian Railways, there is not much we can do here.
However, I suggest you to contact the RhB Railservice, the customer service team for Glacier Express. I know they are great in finding solutions, therefore I am certain they are able to help you:
E-mail: info@glacierexpress.ch
Phone: +41 81 288 65 65 (from the U.S.: 011 41 81 288 65 65)
Perhaps they can reserve the seats and collect the fee on bord the train.
One question: do you have 3D secure 2-factor authentication enabled for your credit card? It is required by many European webshops, even the SBB ticketshop requires it. The missing 3D secure activation is often the cause why credit card payments do fail with the SBB ticketshop.
Kind regards,
Roland