As amavisd is most often chrooted to /var/amavis the process should not have write permissions to that directory for security reasons. A good place for the pid file would be e.g. /var/amavis/var/run
If a user sets a custom path for the pid file the rc script does not respect that as /var/amavis/amavisd.pid is "hardcoded".

Fix: 

. /etc/rc.subr
@@ -20,7 +22,6 @@
 rcvar=`set_rcvar`

 command=/usr/local/sbin/amavisd > /dev/null 2>&1
-pidfile=/var/amavis/amavisd.pid
 required_files=/usr/local/etc/amavisd.conf
 extra_commands="reload"

@@ -59,6 +60,7 @@
 # set defaults

 amavisd_enable=${amavisd_enable:-"NO"}
+amavisd_pidfile=${amavisd_pidfile:-"/var/amavis/amavisd.pid"}

 load_rc_config $name
 run_rc_command "$1"--9cGY082VsFMBF7znWQx2ygurQLxnOen1SGsUImwfBaF7BbkU
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--- /usr/local/etc/rc.d/amavisd.bak     2009-03-12 12:21:42.000000000 +0100
+++ /usr/local/etc/rc.d/amavisd 2009-03-12 12:24:59.000000000 +0100
@@ -12,6 +12,8 @@
 # Add the following lines to /etc/rc.conf to enable amavisd:
 #
 #amavisd_enable="YES"
+#amavisd_pidfile="$PATH_TO_PIDFILE"
 #