The command wc -l ff_invoicepreviewqueue1.csv would usually return count with the file name like, 499 ff_invoicepreviewqueue1.csv. So you cannot compare it with 500. You may do something like cat ff_invoicepreviewqueue1.csv|wc -l which will return only the count.
Also, operator for greater than is "-gt", ">" probably will not work. So to rewrite your if statement it will be:
if [ "$(cat ff_invoicepreviewqueue1.csv|wc -l)" -gt 500 ]; then
# send mail
Remember to close the if condition with fi.