If they have not responded to your efforts to pay an agreed-upon amount for an agreed-upon product or service, you can alert them you'd like to invite them to mediate the dispute in lieu of a court case. This should show them that you want to work with them, together, to problem solve yet will go to court for litigation if they ignore you or fight you. Be professional and polite when you make the offer. Giving an ultimatum rarely inspires people to respond to you, except in anger, resistance or aggression.
Mediation can be attractive because it is confidential, less expensive, collaborative, has a stronger problem-solving proposal dynamic, is more flexible in creating solutions, one can express oneself directly to the other party, can feel more heard and can negotiate as much as mediate.