We already use PayPal IPN to inform our system when they complete a payment for your E-junkie buttons, so we can then process the order accordingly. The IPN details we pass to PayPal checkout override any manual IPN settings (or lack thereof) in your PayPal account, so it won't work to set up IPN in PayPal to notify any third party like Aweber for your E-junkie sales.
Our standard Integration feature is designed to mimic the PayPal IPN format, so services that already support IPN should be able to support our Integration submissions with little-to-no modification. We'd been communicating with Aweber earlier this year on that very topic, so I just now followed up on that to inquire about their progress -- thanks for the reminder.