Shopify offers a flag for each user to determine their preference whether to receive Marketing SMS or not.
Our application offers various ways to send SMS to your customers. Whether or not the content is considered to be Notifications or Marketing is not always clear, especially if the editor allows free text for the SMS content.
The way the Marketing flag is considered or not, is described in each of the following articles:
In cases where you would like to opt-out customers & phone numbers completely from all traffic, you can SMS.to’s Fully integrated opt-out/in management.