A single letter will change each word below into a new one. The letters are different for each word, but they are related. Can you determine the letters to be added and their relationship?

COD
SUPER
MAGI
SEE
FIR
SCAR
ASPIRIN
HEART
TAX
HAD
THAN
PEAR
CHAR
DOZE
LIMB
PLUM
ER
CARES
PUT
LIE
VIE
FLU
PUN
QUART

HINTS:
#1: The order of the clues is important.
#2: The added letters all go in the same place.
#3: The added letter is placed at the end of each word.

Answers:

CODA
SUPERB
MAGIC
SEED
FIRE
SCARF
ASPIRING 
HEARTH
TAXI 
HADJ 
THANK 
PEARL
CHARM
DOZEN 
LIMBO 
PLUMP
ERR
CARESS
PUTT
LIEU 
VIEW
FLUX
PUNY
QUARTZ

The added letters are A - Z in alphabetical order, with the exceptions of Q and V.