Agreement With Corporate Clients And Vendors-reg;

We are a tour operator and event management company,where we provide services like tour packages,hotel bookings, flight bookings,visa,forex to other companies,similarly we take seasonal cost for each services from direct vendors or through 3rd party aggregators. now our major concern are as under :-Q1) what are the key points we should mention to secure payments if we deliver the services on credit (80:20). Q2) what are the points to be mention for secure vendor commitments for any of the above services.?