adding a custom element to paypal direct checkout
This is a bit of a hack I realize but I'd like to add an extra field to the data being sent to paypal that would hold a reference to where the shopper found the store (magazine ad, website, search engine, etc). I was thinking I could somehow tack it on to the end of the merchandise loop in the checkout page where the user is filling out their billing/shipping info. I'm also quite happy to throw it into any existing optional fields paypal direct payment pro has available if that makes it easier.
Anyone out there have enough experience with paypal pro and eCart to guide me through something like this? If possible I'd prefer to just hand code the change in there, I find changing existing eCart configs tends to mysteriously break things.