In answer to your question, as they still have your eqipment in their posession I don't think you have any choice, legally if you want to dispute a bill your supposed to pay it and then seek legal chanels.
You could go along as though your playing ball, all chatty and polite, give them a hand getting your equipment loaded into your van then:
A - Give them a cheque

but accidentally cancel it :

B - Once your gear is loaded, climb into your vehicle to get your wallet before screaming off in a cloud of smoke, take some pepper spray with you in case you get into a fight.
Seriously though in this situation the term 'Short and curlies' comes to mind.
Paul