Consegui substituindo esse trecho do código do plugin (neste caso ficam 12x sem juros ok?)
function francotecnologia_wc_parcpagseg_calculate_installment( $price = 0.00, $installment = 0 ) {
$price = (float) $price;
$installment = (int) $installment;
$result = new stdClass();
if ( $installment < 1 || $installment > 12 ) {
$result->price = 0;
$result->total = 0;
return $result;
}
$coefficient = array(
1, 2, 3,
4, 5, 6,
7, 8, 9,
10, 11, 12
);
$result->price = sprintf( “%0.2f”, $price / $coefficient[ $installment – 1 ] );
$result->total = sprintf( “%0.2f”, ( $price / $coefficient[ $installment – 1 ] ) );
return $result;
}