In your paypal account there is an option for signature.

Obtaining API Credentials

To use the PayPal API, you must have API credentials that identify you as a PayPal Business account holder who is authorized to perform various API operations. Although you can use either an API signature or a certificate for credentials, PayPal recommends you use a signature.

Important:
Although you can have both a signature and certificate, you cannot use both at the same time.

Creating an API Signature
An API signature consists of an API username along with an associated API password and signature, all of which are assigned by PayPal. You need to include this information whenever you execute a PayPal API operation.
You must have a PayPal Business account to create a signature.

To create an API signature:

1. Log into PayPal, then click Profile under My Account.
2. Click API Access.
3. Click Request API Credentials.
4. Check Request API signature and click Agree and Submit.

In IXXO Cart Paypal settings make sure that you have the right URL.

Live API Certificate SOAP: https://api.paypal.com/2.0/

Live API Signature SOAP: https://api-3t.paypal.com/2.0/

Sandbox API Certificate SOAP: https://api.sandbox.paypal.com/2.0/

Sandbox API Signature SOAP: https://api-3t.sandbox.paypal.com/2.0/