If your message is entirely in English, you'll have up to 160 characters so your message will be considered as 1 unit.

Each new line = 5 chars

each link = 17 chars

So Basically a message like this one:

Hi,
We have a cool promotion for you! Get 50% off of our new collection. Click here to claim {link}

Unsubscribe here: {unsubscribe_link}

will be considered as 159 chars which is PERFECT!

However, what happens when you add a Euro symbol for example?

Hi,
We have a cool promotion for you! Get 50€ off of our new collection. Click here to claim {link}

Unsubscribe here: {unsubscribe_link}

This message will be 3 units, meaning that you will pay X3 than the previous example. Why is that? Because a Euro currency symbol as well as many other letters in many different languages symbol make your message to a non-GSM.

In other languages like Hebrew or Arabic which are considered as Unicode, you'll be able to use up to 70 chars per 1 message. 

Emojis are also treated as Unicode, so messages that contain an Emoji - will limit the text to 70 characters.

The best way to deal with it would be to keep it strictly in English - use USD\EUR ($ sign is allowed but can be identified as spam).