You can contact existing customers by using their email address with the order.
Etsy will suspend your messaging privileges if you violating their policies. For example, messaging POTENTIAL customers with coupon offers, asking for their email address or trying to take the sale off of Etsy (Fee Avoidance).
https://www.etsy.com/legal/sellers/#community
c. Communication Standards
i. Messages
With Etsy's "Messages", you have the ability to communicate directly with your buyers or other Etsy members. Messages are a great way for buyers to ask you questions about an item or an order.
Messages may not be used for the following activities:
- Sending unsolicited advertising or promotions, requests for donations or spam;
- Harassing or abusing another member or violating our Anti-Discrimination Policy;
- Contacting someone after they have explicitly asked you not to;
- Interfering with a transaction or the business of another member;
- Exchanging personal contact, financial or other information for the purposes of evading the checkout process on Etsy, including phone number, address, email, social media handles, external URLs, instructions for money transfer, etc.
