I want to create a new contact using the customer email provided during checkout from an eCommerce website. From what I've dug up online, it sounds like this is not possible due to the API lacking server to server authentication. Is this correct?
Even though our Chat Support was able to answer your question on this, I wanted to make sure our response was shared here in the Community as well. At this time a tool like you are describing is currently available for our Shopify integration, but is not yet available for the eCommerce integration. We apologize for any inconvenience this may cause and have submitted this feedback to the appropriate teams on your behalf.
I also wanted to mention that a server to server oAuth is not required for this type of integration. If you set up our current oAuth correctly it will only require the Constant Contact account owner to grant access once and the rest can be automated.