SMS.to is a hidden gem for SMS campaigns, it has all the benefits of the best sms tools out there like upload contact list (like in email marketing campaigns), you pay per SMS not for user (suitable when you have a big ammount of users), SMS price is the cheapest I have seen in the market is not clear how much you pay for SMS because it might depends on the volume. And the best feature for me is that you can pay with paypal and not only with credit cards, having more than one option to pay is for me one of the features a software can have.