Passing product info to PayPal using Comment1 and Comment2
I've been researching this very issue myself. I spoke with PayPal and they told me the following.

First, they recommend you use the PayFlow Pro Module.

Second, to add $comment1 and $comment2 variables to your code you need to:

Create the form fields for COMMENT1 and COMMENT2 to add to the Return Array at lines 293 (of commerce_payflow.module):

‘comment1’ => $_POST[‘your_form_field_here’],
‘comment2’ => $_POST[‘your_form_field_here’],

Note: You may need to build the form fields in the function starting at line 336.

This is really a half-solution, because:

Now, I'm still trying to parse out exactly how to do this myself but I'm pretty sure it involves a custom module that uses a hook to alter a function in that module (possibly commerce_payflow_link_settings_form or commerce_payflow_link_default_settings).