Warning about sms.ac

My son who is away at university suggested we set up accounts on
sms.ac, so that I could text message him from my computer. It looked
to be a good idea, so we all set up accounts (my son, my daughter and
I). We've not used it much, and we were all careful to set the
parameters to not get any advertising messages. This past month I got
2 "I want to be your friend" messages, which I dleted. Then my cell
phone bill came in and I saw that I was charged $0.25/message as
"premium entertainment" - I have a package for messages, but this
charge was extra. I've terminated our accounts with sms.ac -- be
warned about these charges - it's paying for spam on a per-message
basis.