Zalo ZNS
Customers read it right in the Zalo they already use with the shop.
Fits when regular customers message the shop on Zalo; today that is 9 in 10 customers.
Change later add SMS alongside, no need to drop Zalo; about half a day.
Details
Needs a verified Zalo OA and an approved message template, 3–5 days. While waiting, send email to customers who have one (question 2).