Good morning, Roseriver.
(1) I believe that most books call "therefore" either a "transitional adverb" or "conjunctive adverb."
(a) In other words, it does the job of a conjunction + an adverb.
(2) Mr. Lindbergh was very reluctant to have any extra weight on his plane; therefore he refused to carry even a pound of mail/ weight on his plane; he therefore refused to ....
(a) I think most teachers require a semicolon after "plane."
(b) "therefore" connects "he refused to carry even a pound of mail" with "Mr. Lindbergh was ... on his plane."
(c) And as an adverb it modifies "refused."
P. S. Some people would use commas -- it's up to you. Your decision.
on his plane; therefore, he refused. ....
on his plane; he, therefore, refused. ....
I think (only think) that most writers nowadays would NOT use those commas.
Student or Learner