GST registration - Interstate billing.

I am a commission agent based in Kerala. I bill a company in Mumbai, Maharashtra for my services. My annual turnover is less than 7 lakhs. I presently bill them on GST basis.

My question is do I require to bill them on GST basis as I fall below the 20 lakhs turnover mark. Should I surrender the GST registration and move to bill them without GST billing.

Regards
Amrish.