How to file return below such situtation

Dear all

This is Vamsee my question is

one my client still  he has not filed any returns after registrationof GST in July  because financial problems (nature of business is C&Fa (carry foreword agent )  now he is ready to pay all taxes and returns along with penalty and interest.

give me the answer how much he has to pay late filling fee (from July to April) GSTR1 & GSTR 3B   still he has not paid any tax now  he will pay tax along with interest @18% pa

kindly provide your best suggestions.

Thanks/Regards

Vamsee

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