Auto-subscribe Customers to Newsletter

I am redesigning a website for a client with a 17,000+ email subscriber list. He sells products and retreat signups and wants customers’ emails to be auto-imported to his newsletter list when they buy something.

Does anybody know of a newsletter program and integration that will do this? I found one involving Constant Contact and Cart66, but all that does is give the customer the OPTION to automatically subscribe during the checkout process.

Would appreciate any ideas. Thanks!