In your case, it is understood that you sell a pre-developed / pre - designed software. This is considered as sale of goods in GST.
Please note if a computer software is procured off-the-shelf or made available through encryption keys, then the same qualifies to be supply of goods. However, in any other case, supply of computer software is classified as supply of services under the Goods and Services Tax Act.
The registration limit in case of supplier of Goods is Rs. 40 Lakhs, however export are classified as inter-state supplies, and in case of inter-state supplies, compulsory registration is required, even if the turnover limit is not reached, therefore in your case, you are supposed to take registration under GST.
For payment of Income Tax, depending on how you classify your income your return would be accordingly filed. Tax can be paid as per normal route via online challan.
Please advise in case of any other clarification.
Thanks & Regards,
CA Aditya Dhanuka.