Here are two options to send a critical message to a buyer.
1 - To send a message related to a specific order via Seller Central, follow these steps:
Go to the Orders menu, select Manage Orders.
Locate the order that you want to contact the buyer about.
In the Order Details column, click the name of the buyer to open the Contact Buyer page.
On the next page, select a contact reason, complete the message, and click Send
Note: You will not be able to edit the subject line of the emails sent from Seller Central. Amazon will deliver the messages related to completing an order.
2 - You may send a critical messages to a buyer’s anonymized e-mail address via e-mail. Include the word [Important] with brackets in the subject line. The email will not be blocked and you won't receive a bounce back message.
Note: You can use capital or lowercase letters, but the word important must be surrounded by brackets to be effective. For example, the subject line "Need additional information" should instead be "[Important] Need additional information" or "Need additional information [Important]."
Note: Use of [Important] for non-critical messages may result in the suspension of your selling privileges. If you use other subjects, the message will be bounced back if the buyer has opted out. Buyers can also flag messages as inappropriate. Therefore, the content of your messages should be limited to requesting only the information you absolutely need to complete an order.
See sample of messages which you can use to reply to a buyer.
When a buyer does not receive your message
Amazon does not block messages if they are necessary for completing an order. If your message was blocked, it is due to the buyer opting out of non-critical messages from sellers or Amazon treats the selected contact reason as one that is not critical for completing the order.