Hello and welcome to the Community, meadstorage999. There is no messaging feature in QuickBooks Online. It is only possible to send transactions to customers by email (such as an invoice). You can enter a message in the email body before sending the email (select Send in the Action column beside the transaction and a preview of the email will appear). 

Thanks for reaching Intuit support. I'm just a post away if you have further questions or if you require additional assistance.