Mailchimp Integration is an extension of the Payment Button for PayPal WordPress plugin (also known as WP PayPal or WordPress PayPal) that can be used to add the buyer’s email to your Mailchimp audience/list after a payment.
How Mailchimp Integration with WP PayPal Works
When a payment is processed in the WP PayPal plugin the extension will automatically add the buyer’s email address to your chosen Mailchimp audience/list.
How to Enable Mailchimp Integration
In order to enable Mailchimp integration in the WP PayPal plugin, you need to specify your Mailchimp account api key and audience/list ID in the extension settings (WP PayPal > Mailchimp Integration).

Brian says
What other status can I select for a new Mailchimp subscriber?
Noor Alam says
@Brian, There is a “subscribed” option.