I don't understand the question in conjunction with the description you give of the process. When the "Proceed to payment" button is available, you should have the required price listed, in detail, on the same page.
If you are asking about Dollar vs. Euro, then I guess you have to look up the current exchange rate. Of course, if the payment process doesn't continue, this is moot.
When I do it, the website of www.mollie.com
opens in a new tab of the web browser (maybe you didn't see that) and I'm given the choice between several payment options (Paypal, Bitcoin, bank transfer,...)