Great things go together. If you use the excellent Shopp ecommerce plugin for WordPress, you can now have customers added to ActiveCampaign upon checkout (if they opt-in to receive email marketing updates). You can read more about it from the creators at Optimize My Shopp.
The integration uses our API to copy over the name and email of the person checking out on your Shopp-enabled WordPress site. Below are some screenshots of the integration.
WordPress plugin settings:

When someone checks out on your Shopp site, and opts to receive email updates and special offers, they are immediately added into ActiveCampaign as a new subscriber:

Remember, we also include an official subscription form plugin for WordPress.
Let us know of any questions!