Indeed there is no choice on this screen. The method used is the first one in the sequence (the sequence can be modified with the handle in the list view of payment acquirers). Listing all methods would require a custom development.

One thing to note is that you cannot use a "2 steps method" like wire transfer or check because the "pay now" button next to an invoice in the portal will automatically mark the invoice as paid once the payment is made; but with a "2 steps method" you cannot be sure that this is really the case until your accountant has actually received the payment. So it means that you want to list more than one "1 step method" (which are essentially credit card processors). Can you explain why you need to work with more than 1 card processor in your business? It's a relevant question because the reason it doesn't list more than 1 method is because Odoo SA considers this an uncommon situation.