Find the answer to your question
You can use AddMemberMessageAAQToPartner API to send messages to your buyer's My Messages Inbox for the orders that were created within 90 days.
You will need to make sure to set the required tag QuestionType to CustomizedSubject in the AddMemberMessageAAQToPartner API request as below.
| <?xml version="1.0" encoding="utf-8"?>
<Subject>Your item 110093125309 has been shipped</Subject>
<Body>>Your item 110093125309 has been shipped today. Thank you for your business</Body>
<RecipientID>YOUR BUYER's eBay USERID </RecipientID>