You have entered an invalid amount.

The offending line of code is from line 234

if (!is_numeric($donate_amount) || $donate_amount <= 0){
form_set_error('items]['. $key .'][donate_amount', t('You have entered an invalid amount.'));
}

The line of code does not seem to be needed as removing it causes no errors in the checkout, in the prepayment or in payment.

Validation that the donation amount is numeric and greater than zero should occur when the user adds the donation.