In general you can change the fonts for the various elements in the Copy and Pay widget. However you can only use Web-Safe fonts.
This is a general problem with custom fonts in iframes. The font needs to be loaded in the iframe itself, i.e. it can't be inherited into the iframe.
This is also going to be true for @font-face in CSS.
Normally the answer to this is to load the font directly on the server where iframe is hosted. The issue for us here is that this means that we would need to load the font in our server, for security, PCI and practical reasons we can't load this or other fonts into our servers.
Unfortunately for now you should use a web-safe font.