If your schedule is heavy, do not start something you will not have time for in the future. You will be writing to people on death row. I hope none of us will ever need to go through the execution of our friend, but this is a real possibility. Knowing your friend could be taken into a room, strapped down and murdered at the hands of an executioner, is a horrible way to die and a horrific death to deal with. Make sure you have family or friends who understand your decision and will be there to support you if this terrible situation occurs.
Most inmates have little or no financial support.
It is completely up to you as to how much you can or are willing to help your friend. Since you are unable to treat this friend to a special dinner and a movie, be supportive in other ways. In most prisons, sending stamps can be done simply by verifying with a US Postal Outlet, sending them a money order, and when received, the postal system will forward the stamps to your friend. Always check with your friend about the regulations for sending books, tapes, gifts, etc. In most prisons, items cannot simply be sent in, the inmate must be notified and forms must be filled out.