I'm working on integrating PayPal Express Checkout into an existing web application for which I've already setup Google Checkout and Amazon Payments (both SimplePay and CBA). So I'm not new to this.
Everything, including the Instant Update Callback, works fine in a setup that uses Sandbox settings. The Callback works fine both on HTTP and HTTPS in the Sandbox. But as soon as I switch to Live credentials, the PayPal UI stops hitting the CallbackURL and falls back to using the (fall back) shipping costs sent in SetExpressCheckout. Obviously, taxes aren't calculated at all.
I'm using the latest version of the PayPal PHP SDK (version 106.0). The only settings that are changed to switch the setup from Sandbox to Live are:
sandbox to live)I see a similar issue posted on PayPal's Community Help Forum, but it doesn't mention a solution. For some reason I can't seem to post in that thread ... perhaps because it's archived.
Update 9/16/2013: Seems to be an HTTPS related issue. Sandbox doesn't work on HTTPS either, but there's no SSL error when hitting callback URL from browser. Certificate is valid and perfectly acceptable to other payment vendors that we use on the site: Google Wallet and Amazon Payments.
This issue was fixed by: 1. Setting CallbackTimeout to 6 2. Using an SSL certificate that is accepted by PayPal. In our case, the cert was issued by "Go Daddy Root Secure Certificate - G2", which wasn't accepted by PayPal's system.