This is a simple case as far as GST and income tax is concerned.
- Registration tax paid on behalf of client is a purely case of working as an agent. If you have a well documented agreement with client, you need not to charge GST on tax paid on their behalf.
- Other out of pocket expenses, you need to charge GST and it should be part of your revenue. As per GST valuation provisions any expense incurred in rendering service should form part of value of supply, so you have to charge GST on other out of pocket expenses as these expenses were necessary to complete your service.
- You need to charge GST on your service fee, your service fee should be mentioned in invoice along with out of pocket expenses, as per above point.
This will have no impact on your income tax liability, though your revenue will increase by out of pocket expense shown in invoice but you will be able to claim expense against these charge.
Let's understand with an example.
Your service fee is 100 + out of pocket expense is 50. Total billing and revenue will be of Rs. 150.
You can claim 50 out of pocket as expense in your IT filing and your net income will remain 100.
Feel free to comment for any further clarification.