The key to this puzzle is noticing that each of the five-letter words can have exactly one letter removed to form a common four-letter word. This is clued by the reference to “20% off”.
SKIRT – R = SKIT
PLUME – E = PLUM
SPOIL – P = SOIL
QUIET – E = QUIT
ACORN – A = CORN
VITAL – T = VIAL
SWUNG – W = SUNG
NOISE – I = NOST
THUMP – T = HUMP
SHAKE – H = SAKE
GHOST – G = HOST
FLOAT – O = FLAT
CLAIM – I = CLAM
MONTH – N = MOTH
BARGE – G = BARGE
The removed letters spell out the phrase REPEAT WITH GOING. The
word GOING can have the letter I removed to make the answer, GONG.