PayPal checkout's wp_redirect is causing "headers already sent" error

When clicking “Check Out With PayPal” using PayPal Standard, I get a “headers already sent” error on any Cart66 Lite version *after* 1.5.0.2

This appears to be something to do with the plugin’s use of wp_redirect() in cart66-lite/views/paypal-checkout.php

According to PHP documentation, the header() function (which wp_redirect() uses) can’t work after content is already sent to the browser; so when I’m using the PayPal button as a shortcode, I’m not sure how it’s even possible for it to work.

I can’t find any other reports of people having this problem, which baffles me. I’ve tried updating WordPress to the latest version *and* disabling all other plugins.

What could I be doing wrong?

http://wordpress.org/extend/plugins/cart66-lite/