Twilio allows up to 160 characters for a single message segment to be sent. If a message contains more than 160 characters, the rest of the characters after the 160th will be moved into another segment of 160 characters, causing the whole message to be split. This will consume more SMS credits so it would be best to ensure that your messages don't reach that limit, only unless you intend to do so.
Tip: Make sure your link shortener (Bitly, Rebrandly, or TinyURL) is set up correctly and is working. If your shortener is not working, your links may end up being fully displayed which takes up much more characters.
Tip #2: The character count includes also the coupon link so it's good to make the message as short and concise as you can.