Can I pay the contractor the $3500 we verbally agreed upon to replace my sewer? (no contract was signed)

I needed my sewer line on my house repaired. A contractor came to my house and stated they would do the work for $3500, total. I asked them if they were sure, because it was lower than the others that I had contacted, and they said they were. I was on site when the work was performed, and there were no differences from the work we discussed, no surprise complications. No written contract was signed. After completion, they sent me a bill for over $6000, with no statement of what changed. I asked for an itemized bill and they just sent a list of the per hour charges for various components of the work. By their hourly charges it adds up accurately. But this seems totally unfair. I would believe there are laws to protect consumers in my position, that contractors can't simply low-ball estimates to get the work and then charge nearly double after the fact. Any advice? I would like to pay the $3500 amount we verbally agreed upon. But I'm assuming they will then file a mechanic's lien for the remainder they are asking for.