How to email customer outside of Buyer/Seller messaging system?

How do you email a customer outside of the Buyer/Seller messaging system?

I sell some products that ship LTL Freight. Occasionally, the carrier cannot reach the customer with the anonymized Amazon phone number and I cannot get a response through the messaging system. In these cases, I need another way to contact the customer. Is there a way to email the customer outside of the messaging system?

I know Amazon recommends adding [Important] to the subject line for urgent messages but you cannot change the subject line in the messaging system. The only way to do this would be to email the customer directly but I can’t figure out if you can actually do that.

Any help would be great. Thanks!