PayPal in-context bolt-on opens blank popup - asp.net

Question

Having integrated PayPal Express for one of our clients (in ASPX WebForms) and received some under-documented integration steps (https://developer.paypal.com/docs/classic/express-checkout/in-context/integration/) for bolting on the so-called in-context integration, we're facing the following issues:
1) The document asks for the Merchant ID, which, for a sandbox can't be found, wondering if it's the same as the Username in the API credentials
2) I've set up an aspx page for the express checkout to do the posts and gets, which only requires the order id (this would be /checkout_paypal_nvp.aspx?oid={OrderID} below). Judging by the example code PP provided, we're guessing this is what needs to be passed in, but on clicking the 'Check out with PayPal' button, there's only an about:blank popup appearing, and the page doesn't get executed.
Also wondering if 2) is caused by using the wrong Merchant ID in 1), hence the double question (sorry).
Screenshot for ease of visualisation:
Relevant code:
<a id="lnkPaypalCheckout" href="/checkout_paypal_nvp.aspx?oid=81774"></a>
<script>
window.paypalCheckoutReady = function () {
paypal.checkout.setup('{API_username_here}', {
container: 'lnkPaypalCheckout',
locale: 'en_GB',
environment: 'sandbox'
});
};
</script>
<script src="//www.paypalobjects.com/api/checkout.js" async></script>
Many thanks in advance.

Answer 1

Answers to your issues:
The merchant ID (you can found under My Account > Profile > My business info > Merchant account ID) can be used for both production and sandbox environments.
Please make sure "/checkout_paypal_nvp.aspx?oid=81774” returns with the correct PayPal flow url. If you comment out the two script sections which for in-context setup, you should see the PayPal flow being loaded in the full page. That will be your first step before adding in-context setup.