********* Business Behavior- Beware Training Companies they wont Pay after services

we are a small upcoming conferencing company and we run a training on Pre-Stressed and Post Tensioned Concrete Repairs. Ms Yvonne booked 2 delegates to attend the event and promised to make payment with a written email after the event. The reason she could not make payment at that time was because management was not available. There after the delegates attended as per agreement and we as an organisation followed the business processes of this industry. Let them sign in to register and after the event they received 2 cpd point as this course is an engineering course and the 2 representatives were given evaluation forms to complete and rate the services. All was good we thought, until the time for them to pay as they promised then they started raising points as excuses for them not to pay for the service they received. There organisation is taking advantage of us because we are a small organisation compared to them and we need to continue running the business and the invoice they have refused to pay using excuses as there technique's is costing us as a start up organsation. Yes we agree there is room to grow and improve hence we give evaluation forms for all our training's. How are we to grow and improve if organisations big like Bigen Africa are de*****ing upcoming service providers?