Bill releted

Usually for small value (< 10k), cash bills can be generated.

Just keep in mind the provisions of Income Tax Act under section 269ST in respect of seller and section 40A(3) in respect of buyer.

Yes you can issue invoice.

There is no non compliance

I think I've answered this. Let me explain.

There is no restriction under GST Law for issue of cash invoices but there are restrictions under Income Tax Act to receive/pay in cash.

How does your serial number gets affected?

You can maintain separate series of invoices as per your convenience. Law allows it.