For a UuA from WT+ to CW you should be paying the difference in carrier surcharge for a UuA - it isn't a tax difference. I guess you ar starting from the US bdohmen? Starting from the UK the additional surcharge for CW over WT+ for a return to NYC is around £106 so the cost of doing a UuA for a return should be 20k avios and £106.

There isn't an additional service charge doing it online. You can also do a booking with UK domestic connections online and do the UuA at the time of booking and there is no additional service/change fee.

Doing an open jaw booking with a UuA is not possible online either at the time of booking or subsequently. Also doing subsequent UuA on an exisitng WT+ booking online when it has domestic connections is not possible online. For these situations you would have to call. I am not sure about the additional service charge if doing a UuA on the phone.