Commerce Paypal- How can I change the "Order No. at Site Name" string?
All of my payment gateway modules, and my recommended best practice for others', is to include a final opportunity before the payment API request is dispatched for developers to alter the request parameters if desired. How you do that in the PayPal modules depends on which payment method you're using. I'm going to assume this is a PayPal Payments Standard (WPS) initiated payment, so the hook you want is:

hook_commerce_paypal_wps_order_form_data_alter()

If it's one of the others, refer to their *.api.php files to find the appropriate hook, but note that some API requests are run through a helper function in the root commerce_paypal.module.