Sending a bulk SMS (to all your customers or many at the same time) costs 10 cents per SMS sent.
Go to the Marketing functions (right next to Customers) and select "Create Campaign". Here you can first enter an internal name and must select the type "Bulk SMS".
You then have the option to edit the campaign and prepare the text. To do this, go to the campaign and select "Design". Test your SMS by selecting the corresponding link. You will then receive an SMS to your registered mobile number.
Once you have prepared everything, you can click "Start Sending" in the "General" tab. Sending usually takes 1 minute per 50 SMS.