Here’s the best options I’ve seen so far for adding sms to bigred and costumecraze.com customer chat.
Programmable SMS - Twilio Text Messaging for Mobile & Web Apps
Basic pricing rundown is $1/mo for the phone number and .75¢/message (sending or receiving) to/from US phone numbers. Twilio works with a long list of countries, and prices to other countries are more, sometimes 5x or 10x more. But still not unreasonable.