Express Checkout first takes the customer to PayPal's site to select their payment method. Then they come back to your store to confirm the actual payment amount.
If you have coupons or gift certificates enabled, it will allow them to enter those on the Payment page. If those are disabled, it will skip the payment page.
IPN works similarly, but in a different order ... the customer goes to the payment-method-selection first, then the confirmation of final total, then to PayPal. same steps; different order.
IPN requires that PayPal can communicate with your server correctly. If you have followed all the troubleshooting steps in the article I pointed you to, then you would have some debug logs to share explaining what it is and isn't doing -- please post them in the relevant discussion.
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