2002 rear air pipe

I busted one of my rear air pipes last year (rock) and the AC place that looked at it today said there may be dust inside it now and that even a tiny shade would cause the AC to break.

Their suggestion was to cut off the rear ac pipes and weld them shut then charge the front.

Is there any other option? He didnt even think replacing the rear pipe would work as there "may" by dust back there.

They want to charge $350 to do this as well..

Thanks!