I assume it was an expired travel credit partially paid for by LUV voucher. The method of payment should be irrelevant. Southwest will give a free 6 month extension for the full amount.
Correct, $400 of the travel credit was paid for by LUV voucher (which would have expired on July 16th, but since it was combed into this flight credit, its new expiration became yesterday since it became part of a flight credit). The remaining $1,200 is a regular flight credit. They are claiming that the terms state that anything involving a voucher (since the voucher came from a travel credit 6 month extension already) cannot get anymore of an extension.