I am very sorry to hear your snake died.

It's actually quite nice that the pet store is going to have a necropsy done. Please note - IBD can not be diagnosed with the general necropsy. Please make sure they do a full pathology testing of the various tissues as well (brain, liver, etc). When I had this done on two of my boas, the tissues were sent to UC Davis as it wasn't something my vet could do in clinic. It took about a week or so to get the results.
IBD is nothing to mess around with and like others have mentioned, there are many things that can cause similar symptoms.