Hi,
The "YYYY payment notification: payment cancelled" email means that either the payment gateway couldn't finish successfully the transaction, or that there was a problem with the transaction details. For example, if the amount paid by the customer is different than the amount of the order in HikaShop, HikaShop will cancel the order so that you can take the appropriate measures as either the customer tried to mess with the system to pay less than required for his order, or something is not configured properly.
Also, when this happens, the PayPal Checkout payment plugin will log details about the issue in the "payment log file" available under the Files section of the HikaShop configuration. So please check in there. If you could provide its content via our contact form, we could check the log to figure out what's wrong.
www.hikashop.com/support/contact-us.html
Having a typo in the product code won't change anything.
However, can you check the transactions on your PayPal merchant account ? Are the payments valid there ?
Could you also provide a screenshot of the settings of the PayPal Checkout payment method ?