Before you start using the MessageMedia app for Salesforce, there are some really important things to be aware of so you don't get any nasty surprises or headaches along the way.
The cost of your messaging depends on how many messages you send, and the number of messages you send is influenced by how many characters you use to write them. So, it's essential that you understand the circumstances where SMS character limits vary.
You can read about this in detail in this article, but putting it simply, if your message contains "special" (aka Unicode) characters (e.g. Emojis), then the character limit per message drastically reduces.
When you're sending messages to a contact via the Send SMS button or the SMS Message Feed, the app is able to detect Unicode characters as you're writing your message and will alert you:
Important Note - as the character counter does not display the number of SMS units/credits required to send the message, please be advised that the inclusion of Unicode characters may increase the number of SMS units required to send the message from one unit to two.
So as you can see, you really need to pay attention to the content of your messages and be very clear on the character limits involved. Otherwise, you might get a nasty shock when you receive your invoice 😬.