Ask HN: Anyone doing anything with SMS coupons?

I've been talking with a convenient store owner who is doing an SMS coupon program which is working pretty well. We have been kicking around some ideas on how to expand it and I was wondering if anyone here has any experience in this domain? What are the major pitfalls? Looks like I can do 2 cent texting with Twilio and it costs about $12K/year for a short code. What else should I be weary of? Thanks for any info.