One UPC = one item. For the purposes of inventory reconciliation you are requested to treat all paired, grouped, bundled, etc, items as a unit or group of units where the quantity thereof is to match the number of uniquely scannable UPC codes, or where multiple codes are affixed to a single saleable unit the saleable unit itself may be considered to be the saleable unit.
Yeah, just the standard ones. Yeah I think that's a great idea. In fact just yesterday I bought some new slippers from the supermarket which turned out to be badly designed chinesium prototype slippers and thought about just having a second pair of crocs for indoors. I just checked and you can even get fluffy lined ones!!