Well, after a brief period of panic, it turns out that because our flight is a joint venture between KLM and Delta, we actually needed to check-in via the Delta website, not the KLM website as indicated by the email. All seems well now, but I feel it necessary to call attention to an quirk of this system. The check-in process allows one to select the number of checked items and to declare any that may be "special". This line in the list of special items caught my eye:
Fragile/bulky items: Saddle, antlers, Hawaiian pineapples, military duffle bags