You can purchase SMS credits in the 'Billing' section under your user profile.
User Profile > Billing > 'SMS Credits'
Is there an expiry date for SMS?
No, your SMS do not have an expiry date.
How do SMS bundles work?
You can buy SMS in bundles of 100, 1000 or 10,000.
When 1 SMS notification is sent or received, your SMS balance is automatically adjusted by 1 credit. Please see image below for SMS pricing.