The issue that I see here, is that a fee is paid twice. Once from the end-user to the IPN provider's unique BTC address, and again from the IPN provider to the merchant's wallet address.
In the simplistic implementation a fee is paid
three times: Once by the customer, once to move to the merchants wallet, and once more by the merchant when moving the funds to an exchange (or aggregating into a storage wallet.) Your idea drops that to two fees.
But there's simple optimization (without unique payment amounts): The payment processor can wait for a day/week/month/hour and aggregate all payments in one transaction to the merchants wallet. That's two fees per payment, plus only one fee to move the bulk payment further. Thus, your idea doesn't gain much.
Actually, if the merchant provides the payment processor their xpub key, then the processor can direct all customer payments directly to an address controlled by the merchant. Thus, there are no extra fees. (This does create the issue of how the the processor gets paid. They can no longer take a cut from every transaction. But they could bill monthly.)
BTW There are two ways to embed a unique amount in a transaction: The amount paid, and the miner's fee. Both can be adjusted by a few satoshis.