Unprofessional service

Our complex had these people quote to fit PVC downpipes to out gutters as we had the asbestos ones removed. On 3 occasions they were asked via email correspondence to confirm the quote. Upon completion of the work, they then said that they made an error and invoiced us for almost double the amount of the original quote, which as per our lawyer we refuse to pay. Be warned, insist on an invoice before paying your deposit. Highly unprofessional service