Corporate air ticketing credit clause how to add

I used to run a corporate travel business few years back but I lost a lot of money because there was no paper work.. no contract.. everything was going on good will & trust and i was never in a position to ask my corporate client for a contract fearing of the risk of competition. Now I have decided to again step into this business and my question is that "I've started pitching my new travel agency to corporates, but the challenge i'm facing is that the corporates do not want to sign the contract initially they say that first let us see your service and then we will think of contract. Is there any legal clause or term i can ask them to acknowledge over email before investing money in their ticketing till the contract is in place?