Postcards basically looks just like Textable, but is free.
OK huh - free? that seems unexpected... but, I'm guessing SMS between carriers is generally free nowadays... soooo....
You still pay for your SMS usage
In that case - Assuming all of your customers are on carriers that have an email to SMS gateway - you can do this completely free through email.
Now to discover that, you'll have to do some work - find what carriers are in use by your customers, then check for a gateway... that could be time consuming.. but hey - could really pay off with a free solution.