If you want to send a personalised message, send one. "Dear Buyer. Thanks for your recent purchase from me. Here is your tracking number, as was already available in the automated message sent by eBay. Your package will be sent via Swift and Shift Couriers and may or may not arrive. Lots of warm bear hugs and kisses to you. Signed <your name>" . There! 🙂

If I want to send a buyer a personalised message, I send one. Often I will send a message thanking them for their prompt payment and I will have their package in the mail first thing tomorrow, or later today (depending on the time they pay). If they live interstate, I will give them a rough estimate on how long it might take for their package to arrive, going on past deliveries.