(a) This would be admissible because only one writing discusses it.
(b) This evidence definitely would not be admissible because it concerns a term with respect to which the parties agree in the writings. If they had really agreed on $10.85, it is highly unlikely that both sides would have made the same mistake in their respective writings.
(c) This seems similar to (b), but I think it is distinguishable and would be admissible. The discount is conditional upon payment by a certain date. This discount thus doesn’t contradict the agreed-upon price term.
(d) This evidence definitely would not be admissible because the phone conversation took place after the purchase order and acknowledgement were dispatched and received.