They could use Continuity-type stuff to have you verify the payment from your phone, that's happening in your desktop browser.

Which sort of makes sense with today's announcements, they said they had the ability to now detect between taps and presses with the Apple Watch, that technology seems like it could be easily integrated under a trackpad as-well...

Apple trackpads can already distinguish between a finger touching them and being physically depressed, though I don't know what it has to do with payments.

Thats a very different mechanism.

Of course, but it accomplishes something similar. Also, trackpads already have the two finger tap for additional actions (i.e. right click menu), and I still don't know what this has to do with the topic. :)