Thank you for the post. While we do not currently provide an Opt-In email option, based on our understanding of eBay policies, anyone who orders from you on eBay is opted in for marketing already.
This means that any marketing emails sent from ShippingEasy, for your eBay customers would be ok. Customers could then use the Opt-Out option that is included with your ShippingEasy marketing emails, to opt out, if they prefer to no longer receive those emails from you.
You can, however create a custom automated email campaign and set your auto campaign rules apply to orders from your eBay store. This would then send an automatic email, inviting your customers to Opt-Out using the link at the bottom of the email if they don't want to receive your campaigns/newsletters.
This link explains: How to: Create a Campaign template if needed.