Adding a substitution drop down or checkout box?

How would I add a drop down or check box to my shopping cart on one of the pages? Quite often we run out of certain items, and allowing online customers to say whether substitutions are ok, would save a lot of extra time. Thanks

There's nothing built in to Zencart that will do that. You might be able to modify this mod:

http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=309

I don't see how I could use that for the substitution, since the substitution would be when we're packing up the order.

I used a mod that added a dropdown box on the Credit Card enter page that lets people choose which card they are using, even though they didn't need to, I was thinking about modifying that mod, but can't seem to find it again.

I guess I misunderstood. I thought you meant you wanted customers to approve specific substitutions, but it looks now like you want them to approve substitutions, before the fact, sort of globally? Like 'if you don't have what I order, send me something else, you choose'?

Yes that's correct. We sell wine both in our store and from our website. Sometimes our inventory runs low or an items vintage (year) changes. And instead of contacting the customer about a substitution, it would be nice to have the customer tell us while they are checking out if a substitution is ok, so we can just process and ship the order.