The plugin does not check to see if it has been selected for payment. To fix this, add the following to line 58 of /secure-trading-gateway-for-woocommerce/assets/frontend/js/main.js
if(('.payment_method_st_gateway').is( ':checked' )) {
and a closing brace to line 75
}
The following can also be implemented in main.min.js but will require you to know what you are looking at.