Business security of payment - Some of my clients want to pay 20% advance against LR copy

Hi , i made a proprietor firm to supply agro products . And i have clients from other state those i don't know personally. But they do have GST number, while getting order(PO) i asked them to show me turnover and annual report and they denied .they said they can show if i meet personally.
Some of my clients want to pay 20% advance against LR copy and some wants to pay nothing as advance. Rest amount (either remaining 80% or 100 %) they want to pay by RTGS when i diliver goods. And forgot to mentioned that they are providing PDC or blank cheque for my security , which they will take back on delivery after paying by RTGS . Now my main question is - my payment is secure from buyer? And i am making an agreement for everything with terms & conditions.

You can negotiate the bills through your banker.
On making payment the bank will release documents for collection of item.
Once both develop mutual confidence and trust, payment terms can be settled.

Shri Nathrao is correct. It depends upon as to what types of goods you are selling? If it is an monopoly item, you can insist purchaser for L.C. Payment.
A. Prakash