Payment/purchase reference

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I am using a Buy Button. I need to send a payment reference with the button. When the customer is logged in on my site and pushes the button I need to know which customer was that. I know that on my web site and I need to send let's say CustomerID to the Buy Button so when the customer pays the order I will see that in an email notification or elsewhere so I that I can ship the product to that customer. I don't want to show this information to the customer. I realize I can set the email of the customer but they might change it, this is why I need another hidden field that only I can see not accessible to the customer.

Another question - once a payment is successfully processed can I specify a URL link to be called to notify my web site so I can reflect the payment as paid on my web site. Also I would be interested if this is not done with a URL but with an API. What should I look for in the API to implement this functionality?