How can I link to a product at Amazon.com by UPC?

You can use the "Link to Search Results" tool in Associates Central under "Build Links" / "Link to Any Page...."

Enter UPC in the "Keywords" field and any text in the "Name Your Link" field. Click "Get HTML."

Copy the URL from the resulting HTML. It will be something like:

http://www.amazon.com/gp/search?ie=UTF8&keywords=UPC&tag=YOURTAG&index=blended&linkCode=ur2&camp=1789&creative=9325

To create a link to a product by UPC, replace UPC in the URL with the UPC of the product:

http://www.amazon.com/gp/search?ie=UTF8&keywords=086892722831&tag=YOURTAG&index=blended&linkCode=ur2&camp=1789&creative=9325

Using Amazon's web services, you can write a program to search for products by UPC and get information about the product, including a link to the product's page at Amazon. The example code at PHP A2S Demo 1 can be modified to perform a lookup by ASIN. Change the IdType in $url to UPC and specify a UPC code instead of an ASIN in $ASIN.