title: PayPal blocking duplicate invoice id in relation to what?
body: <p>According to the <a href="https://developer.paypal.com/docs/classic/admin/setup-account/#id095RE40H05Z" rel="nofollow">documentation</a>, PayPal can prevent duplicate purchases by blocking a payment when the <code>invoice</code> field's value has already been used in a previous purchase. My question is, <strong>used</strong> <code>invoice</code> here is in relation to what - my merchant account as a whole, or only for the current <code>item_number</code>?</p>