The package Dr. Reynolds is carrying most likely contains some kind of payment. The book never specifically says.
In chapter 30, Jem and Scout have been attacked been Bob Ewell. Jem is lying injured, and they call Dr. Reynolds. When Dr. Reynolds leaves Jem’s room, he notes that Jem is alive and kicking (literarily)!
Dr. Reynolds was carrying a big package wrapped in newspaper. (ch 30)
The package is not mentioned again. Earlier in the story, it was mentioned that the Finch family does not have much in the way of money because most of Atticus’s clients pay him with trades of whatever they have, such as chickens or other food. Chances are this package contains something that the Finches are using to pay the doctor. Since it is wrapped in newspaper, it is not likely medical equipment.