Shell Programming and Scripting

Hello,
I have written a script (say chld.sh). its structure is as follows:

------------------------
#!/bin/sh

usage()
{
echo "chld.sh <param1> <param2>"
exit 0
}

chk_param_1()
{
case $param1 in
c) export PATH=$PATH:/home_a/bin/execs;;
d) export PATH=$PATH:/home_b/bin/execs;;
*) usage;;
esac
}

chk_param_2()
{
echo "Chk Param 2"
}

clear
param1=$1
param2=$2
chk_param_1
chk_param_2
------------------------

Now I source this script, as I want the new PATH variable in my current environment. Hence I give a alias in my environment as
alias set_path='. ~/chld.sh'

Now when I call the script as:
set_path c ........it works fine
set_path d..........also works fine.

But if I give "set_path e"......it displays the usage and EXITS/LOGOUTS from my parent shell i.e. the main shell.

I know the reason is coz due to sourcing, it runs the chld.sh in the current shell and not another subshell.
But please tell me how to do this.

Whenever I display "usage".....i dont want to continue ahead in the script.
Simply come out of this script.

Please HELP!!

Well return works amazingly well.

Thanks a lot.

Mods....please close this thread as the answer given is the best!!