Thank you for your reply.
We have a store that sells software. We apply discounts to some titles if the customer owns an older version. To qualify for the discount, he must input his serial number, which gets verified via an API to another server that keeps track of serial numbers.
I can handle all the serial number lookup stuff, I’m just not sure if the custom product options is the right mechanism to use in Magento. I need the serial number to be stored with the completed order. I want the user to enter the serial number in a custom input box right in the shopping cart page /checkout/cart/. I do NOT want the option to be available in the individual product page, where options are normally entered. This is because I have special logic that determines if the serial number is valid, and I’m not sure how to add this validation to the product options if they are entered in the normal way on the product page.
I can probably do this by customizing the individual product page to hide/suppress the serial number option input, but I’m wondering if there is a better way.