n. 25B6; adjective extra income : ADDITIONAL, more, added, supplementary, further, auxiliary, ancillary, subsidiary, secondary.
25B6; adverb
working extra hard : EXCEPTIONALLY, particularly, specially, especially, very, extremely; unusually, extraordinarily, uncommonly, remarkably, outstandingly, amazingly, incredibly, really; informal seriously, mucho, awfully, terribly; Brit. jolly, dead, well; informal, dated frightfully.
postage is charged extra : IN ADDITION, additionally, as well, also, too, besides, on top (of that); archaic withal.
25B6; noun
an optional extra : ADDITION, supplement, adjunct, addendum, add-on.
a film extra : WALK-ON, supernumerary, spear-carrier.