Originally Posted by MSPeconomist

$400 sounds like a lot of APD, assuming one person, especially since the original ticket apparently included the other taxes and fees. What cabin class/fare booking class are you using?

I'm suspicious that $400 is a fare differential, although the agent could be adding a stopover fee (often $100 on international tickets IME).

Can the OP look at the fare rules to see whether "stopovers" (which this wouldn't be, but if the agent thinks it is, that agent could be attempting to reticketed in a higher fare class that does allow stopovers) are allowed and whether they're associated with an additional fee.