Well I think I may have figured it out, Square may want to look into this as it appears to be related to the dreaded gremlins caused by LOCATION edits.
Any of my items that were created with a [reusable variation set] were somehow - behind the scenes - linked to an OLD LOCATION we had created for our account. This OLD LOCATION was not visible as 'inactive' on the Locations listing, however it still showed up in a drop-down list as a pickup location. This obsolete location was also permanently associated with our online web site and even when I tried to change to the current location, it would revert back to the obsolete phantom location. This caused all sorts of problems on our web site - items not going into the cart, items not allowing for selection of sizes in the checkout workflow, and 404 page not found errors.
There was no way for me to edit or disassociate the obsolete location from the item variants, so I had to completely delete the items that used the reusable variant sets and re-enter them into my item library. It would be really good if there was a bulk edit feature that would allow the user to at least view associated locations but ideally edit, so that when a location needs to be changed the items can be updated.
But the phantom obsolete location hanging around was the big problem here. I never could detach it from the items so I just deleted and re-created my items. At least our customers can check out again. Good thing we don't have too many items in our library! I only wasted about 16 hours on this problem, which all began when we needed to add a pickup option to our products.
