i hope you found a solution to your problem already; in case you didn't, and for others who might face the same problem, i want to describe how i solved it:
the ccavenue payment module passes the total purchase amount as a number formatted with the PHP-function number_format to the ccavenue web site. as a result, the amount gets formatted with a ',' separating groups of thousands (1,234). ccavenue doesn't seem to like this comma and returns 'invalid amount'.
i changed the following code in /includes/modules/payment/ccavenue.php (line 157, in my case) from :
zen_draw_hidden_field('Amount', number_format($cOrderTotal, 0)) .
which formats the amount without comma, and ccavenue stopped complaining.
zen_draw_hidden_field('Amount', number_format($cOrderTotal, 2, '.', '')) .