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 and the maximum amount of chars will be 120.

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).