There are four backroom robbery missions you can trigger. Take note that the option to rob the back room or basement will not be available until you observe the shady activity. Consider the following tips before performing these robberies:
At the back of the drugstore, you can peek through the window or knock on the security door to learn about the shady dealings behind the drugstore. Prepare your weapons and make sure to hitch your horse behind the store so you can quickly get to it when you’re ready to escape.
Wear your mask and enter the store. Aim your weapon at the doctor then press UP in the DPAD to order him to take you to the back room.
As soon as the door opens, head inside and take out the three gang members, including the woman. Don’t bother looting the corpses; your priority is to get the Schofield Revolver (best revolver in the game) from the weapon case and the money clips on the table as well. The sheriff’s office is right beside the establishment so make sure to leave the scene quickly. Avoid killing anyone else, including the responding lawmen if you want to retain your honor and prevent your bounty from racking up too much.