To stay logged into the POS after finishing a transaction, you can just change a setting in the positions setup screen.
Use the left side of the window to select the position you would like to stay logged into POS. Then, with the position selected, expand the Point Of Sale heading on the right using the [+] button next to it.
Now UN-check the box labelled Prompt for Relogin. This will remove the prompt asking you to log back in after every sale.
Press to save your changes to the position.