If it's a designated sweep flight there is no extra pay.

If it's not, you'll have to submit a pay log.

Interesting note: I was launched on an airport stby (not on reserve) and the pairing we stepped to the jet with was a simple PM O&B that was a late minute XTRA pairing. At the first destination, we were rescheduled to fly to LCK before returning to MEM. Given the original pairing was just an O&B, and I was not on reserve, the company had to pay for xtra landing --- I believe in the new contract it's just +30 min though.