A setting to customize the string to append to the usage SMS message as been added in version 1.21.16.
Go to Setup, Notification and you are able to edit the SMS suffix.
“Enable Customer Topups via SMS Reply” must be on for customer replies on SMS messages to trigger a topup on their account.
This SMS topup reply suffix is added to usage SMS notifications. Data usage notifications must be on for these SMSs to be sent out.
See post on how SMS reply topups work: SMS Topups: How To Do Automatic Topups With SMS