a) You must have a strong knowledge of most points and miles programs

"Understanding" would be a better word to use within the sentence structure.

b) shake things up a bit with this application process( 

A colon is the right punctuation mark to use here.

c and d) numbered liss with “I Included ALL 12 items” as the subject line. 

"List" is misspelled.

Suggestion: Either capitalize all words in the subject line, or only the first word; don't mix and match within the same sentence.

e) If you’re interested in full-time work, add “- FT” at the end of th subject line. 

The second "the" is missing an "e" at the end.

f) Your three favorite frequent flyer programs, with a one-sentence explanation for why you love each

A comma after "programs" connects the the two independent clauses.

g) Your three favorite credit cards, inc1uding a one-sentence explanation for each

The numeral "1" in the word "including" should be the letter "l" instead.

h) A list of three points- and miles-related features you’d like to write that have never appeared on TPG

A hyphen after "points" clarifies that the features should cover both points as well as miles. In its current state, the list asks for three points, and an unknown number of miles-related features.

j) You may only send your email application between the hours of 3pm and 6pm ET on Sunday, June 14 and Tuesday, June 16.

The wording creates ambiguity about whether the application time is a three-hour window on Sunday, and again on Tuesday; or if the timeframe commences Sunday at 3 p.m. and ends at 6 p.m. on Tuesday. 

Nerd bonus: The AP Style Guide favors formatting times as 3 p.m., 6 p.m.