I came here with this same question. I never saw the Comments section. I went back through check out to see why. On the page with the notes section, Paypal AUTOMATICALLY scrolls to the bottom of the page to the Send Payment Now button. I just did it 3 times in a row. This is what I saw each time. If you want to put in a note you have to scroll up, which isn't very intuitive. You'd think the note section would be between the total and the payment button. I hope this helps people out with Paypal's terribly designed Payment page.