I made a purchase from a website the other day. Like I do with most online purchases, I checked on the package tracking. This particular website uses the ups tracking api (is there one?) to display the tracking information on their site. on the tracking page they display related products that are also for sale. I'm sure that a lot of sites do this it's just the first time I've ever seen it and if your site is not doing it you should be! The order I placed on this site is going to take about 5 days to be delivered. I check the tracking at least once a day (mostly just because I think it's neat to know where my package is, I'm sure others do to) so this site is going to have at least 5 chances to sell me something else at a $0 customer acquisition cost (assuming the ups tracking api is free)!