Originally Posted by
Winglets747
This I don't know about, but I want to stopover in AMS, not LHR in one direction. I would think adding LHR-AMS-LHR would have a minimal additional cost as going CPT-LHR-AMS (stopover) AMS-LHR-JFK and then back to JNB with no stopover in Europe would have a minimal additional cost as the fare might be higher but I wouldn't be paying UK taxes, but it's considerably more expensive.
I'm afraid that was wishful thinking. It looks like you may be running into problems with combinability when you try to write this all onto one ticket. The lowest price I can see adds a CPT-AMS fare to an AMS-JFK fare to a JFK-CPT fare, and all of the three parts is more expensive than half of the CPT-JFK-CPT fare.
If you want to do this, you may want to buy the CPT-JFK-CPT fare and then buy a separate ticket for LHR-AMS-LHR to get the stop in AMS. However, you will not save on UK APD this way as the calculation will treat these as the two separate tickets that they are.