SKU numbers? Amazon Checkout

I am using Amazon Checkout v 1.0 on ZC 1.5.0 and when I place a test order, the order comes back to me with a SKU number assigned to it (usually something like "3")... I asked their help people about it and their response was:

"Amazon programs, including Checkout by Amazon require SKU numbers for all of the products. You will want to contact Zen Cart for assistance in updating the SKU numbers being assigned to the products as it is not being added on Amazons end."

So... where is it set? Where is it coming from? How can I assign them?

I figured out--it's using the ID number of the product... pretty much the order in which you added the products to your catalog determines that number. So that's your SKU with Amazon.