Please use code blocks. They're so much easier to read.
I still vote for 'rm -rfi', but this is (arguably) a mild refinement of wpeckham's solution. This does test to ensure subdirectories are not deleted, if you care. Since it runs as a new process, pushd and popd are superfluous.
if [ "$1" == "-a" ] ; then
if [ ! -d $dustbin ] ; then
echo "dustbin does not exist or is not a folder."
for foo in * ; do
if [ -f $foo ] ; then
if test $ASK ; then
rm -i $foo
rm -f $foo
echo "$foo is not a file I can delete."
echo "Done cleaning dustbin."