Does your PPC program drive more sales than it's credited for? The answer is certainly "yes", but the sources and volume of under-reporting might surprise you.
Knowing this, you might say: "Well, if I have a sense that this happens 20% of the time, can't I just adjust my advertising efficiency thresholds by 20% to compensate?" Yes, you can, but the problem is that some destination pages are more susceptible to this problem than others, either because they load more slowly, or because users are more likely to navigate off of them quickly. This will disproportionately penalize some types of keywords over others resulting in lost opportunity as those ads are mistakenly bid down the page.
A better way to track high-dollar marketing programs is through use of a fast redirect. The redirect is fast if, and only if the redirect server doesn't have to do a database look-up. If the server has to look up the destination url the redirect will be slow and the server will bog down during traffic bursts. We pass the final destination url to our redirector as an encoded parameter so the redirect takes less than 0.1 seconds and the volume of redirects is almost irrelevant.
Using a redirector provides much more robust tracking, but can/should be cause for concern as well. With all of that valuable traffic passing through a third party box it's valid, indeed essential to ask: "what happens if the box goes down?" We stressed out about this, too. Our approach was to build in multiple redundancy by having multiple redirectors. To keep these independent, these servers are located across the country, and use different internet backbones. All the servers share the work, and are self-checking and self-correcting. If a data center becomes unavailable -- say, due to a server failure (almost never), or due to DC connectivity problems (very rare), or due to routing hiccups somewhere on the web (not rare, but brief) -- we use smart DNS to reroute traffic to the healthy machines within a minute. No system is 100% perfect, but this redundancy and automatic checking provides extremely high uptime.
Handling order allocation issues can be done on the fly, with the confirmation page tag sending marketing allocation along with the order details, or through a back-feed of order ids and marketing channel credited. Any competent search agency can then base its bidding and reporting on only those sales your system has flagged as paid search or unknown.
We see all the costs associated with PPC advertising, but we don't see all the sales generated. While this post may cause some eyes to glaze over, understand that this is not minutia by any stretch of the imagination.
Better tracking technology plugs a big hole, but others remain. Users drop cookies, use multiple browsers, and sometimes search on one machine but place the order on another after shopping around. We can track spillover to the call center, but measuring foot traffic driven to the stores remains elusive. However, those who throw up their hands and conclude that direct marketing metrics shouldn't be applied to search simply aren't trying hard enough.