The tracking is viewed on the avast anti-theft website, whether you start the ‘request location every x mins’ via that website or SMS.
The device being tracked doesn’t send an SMS with the GPS location coordinates, it displays it on the avast website. The device must have an active data connection, Cell or WiFi.

Apologies I just realised you was asking for SMS as a means of communicating the devices position. I got confused by you being concerned about the excess SMS sent.

I agree being able to get an SMS from the device with its gps location would be of great benefit when the device is not on an active data connection.