Gst billing

Hello

one of my client have not knowledge of computers and I can visit there once or two time in a month.

I want to know that can he issue manual bills (hand written) on a/c of supply made by him? After that I will enter that bills into system.

Further he is dealing in mobile store and selling mobile phones and accessories.

Can we keep two series of billing. one for mobile phones and second for accessories??

Pls help.. Thank you.