Export of Service

Yes GST is applicable as the turnover exceeds Rs. 20.00 lac and accordingly mandatory required to register under GST law irrespective of the fact that even one transaction is carried on. As for terming any activity as business activity, continuity of activities is not required even one single transaction carried on would suffice as in your case.

As you are exporting services, you can claim refund of GST provided all the required conditions are fulfilled.

Hence you need to register under GST law and file returns on regular basis.

As you said you are in software services, so check the place of supply in order to treat the services as exported. 

Conditions for terming services as export:

1. Location of Service Provider is within India.

2. Location of Service Recipient is outside India.

3. Place of Supply of Service is outside India.

4. Consideration is received in freely convertible foreign exchange

5. Service Provider and Service Recipient are not merely the establishments of distinct or related persons.