You can only send mass text messages to customers who have given opt-in permission. And more specifically, you can only send them the types of messages they’ve opted in to receive (such as shipping notifications). If you want to send recipients a marketing message, you need to get explicit opt-in for those types of messages as well.
If you don’t respect recipients’ opt-in preferences, you could be flagged as a non-compliant sender and carriers may not pass through your messaging traffic.