Advise on cash sales

There is no cash invoice or credit invoice as such under GST . It is only supply invoice under GST or Non supply invoice.

When ever there is cash advance received , you need to charge GST rate on the advance also.

Only under IT rules below Rs 2 lakhs restriction in cash.

When the material is billed and despatched the GST is applicable

What about Pre GST? Under VAT structure. If goods take 4-5 days to reach vendor, can he Cash Invoice the customer without receipt of goods?

Tax invoice is only to be issued whether cash or credit sales. In case of credit put their name on invoice.