In your Account Settings under Billing, you will see 2 billing fields. "Billing" on the left is the payment details for your platform license fee. "SMS Billing" on the right is just for your SMS costs. It's important that between organisations and their artists, that the correct methods of payment are entered here to avoid confusion. You will be directed to Stripe to complete the payment setup steps.
Once you've added your SMS Billing payment method, you need to add credit to your account before you can send SMS Broadcasts. To do so, follow the below steps.
Navigate to Engage then SMS Inbox.
Then click on top-up your credit, choose an amount, and this will be taken from the payment method you added in the previous steps.
TOP TIP: As you draft an SMS broadcast, your total cost of the broadcast will be tallied on the right hand side of your screen, which will inform how much you need to top up your credit by.