the error being returned is:
Security header is not valid
This error means that the API credentials being used are not correct.
i see the username you are using is an email address:
<Username> info@ds-ergonomics.com</Username>
this is not a correct API User name.
The User Name, Password and Signature you should be using is the API Username, API Password and API Signature that is listed on the PayPal API Credentials page.