@jcharris,

try this, also be sure that you have the customer message enable under settings > checkout > Email receipt.

function jch_test_function( $message, $email_tags ){
global $ImStoreCart;
$message . print_r( $ImStoreCart->cart, true );
return $message ;
}
add_filter( 'ims_customer_message', 'jch_test_function', 50, 2 );