I'm looking for a Wordpress plugin or other means to turn a shopping cart into a rental system. A friend of mine is launching a piano rental company and wants to place the entire process online.

Customer will choose piano, length of rental, delivery options by zip code, e sign a rental agreement and check out. A copy of transaction goes to the piano delivery company. Any suggestions would be greatly appreciated. Thanks

Hi and welcome, networkordie!

Give the Booking Calendar plugin a try:
wordpress.org/extend/plugins/booking/

Hopefully it will work out well for you.

Kind regards,
Nick