There actually lots of "online rent payment" services now. You can also look into these options:
The other method is to set up a separate bank account for your rental income/expense only. Your renters can direct deposit their rent payment into the account on a specified date. It can be just like any other direct payments you make (car payments, cable TV, mortgages, newspaper, etc). Most of the banks have those services either free or they charge a very minimum fee.