INF/NS have their own pay scale, but it's NOT 1/2 of BE - and yes, the INF was the reason you couldn't get the OP-Up (No INL complimentary upgrades) as the INF ticket would have had to been priced and reissued at the current fare.

INF can be added at anytime to a domestic, even at check in, but it's SO much better to have it done right away, as has been said, on certain aircraft there are only some rows with the additional required O2 masks, and there are restrictions on how many INF/NS in a row (on Domestic - it's decidedly all about safety).