The parking lot is small but usually the turnaround is quick enough that you just need to wait a few minutes for someone to leave.
There can be lines around the holidays or towards closing time, but the few times I cut it close and showed up 5-10 minutes before closing they stayed open a few more minutes to make sure I got my package in.
The officers here have always been courteous and helpful, even taking some extra time out of their way to help me figure out the best deal for mailing my package the quickest and cheapest way possible.