Receiving Access

We are having an internal discussion about the level of access we allow vendors on our Receiving dock.  I have polled some local co-op's and they differ from stuffing a piece of cardboard under the door and allowing access for all, to buzzer entry only.  Currently we have a buzzer entry and a keypad entry option, but both systems are tired and in need of repair.  With a busy downtown location and increase in bad behavior all around us, we are trying to balance safety & security with open access to a variety of different vendors.  Can anyone share their current practices for us to consider?  Thanks for the help.