Cardholders have two years from completed delivery of product to issue a chargeback and you cannot issue a partial chargeback, it's all or nothing. I own a merchant services company and can tell you that if the merchant has had problems with this type of thing, they should become more cooperative if you threaten a chargeback. Things like used car lots are very challenging to underwrite for a merchant services account due to this exact type of problem. Multiple issues can result in the merchant having funds held or their being placed on the ban list (called Match). If they are using a fake merchant account like Square or Paypal, it can get really ugly in a hurry. So, I would let them know my intentions and see if they'll help. If they haven't had problems, they may be oblivious to the potential problems in which case a chargeback could be needed. Like I said though, a CB is all or nothing, so you'd have to take the car back a CB the whole thing, can't just do the cost of the AC. Good luck.