Sorry Lisa, I guess I did not quite understand your initial question regarding cancellations.  I would suggest you not do work for companies that are pulling that kind of scam on you.  Some cancellations are acceptable to me, but not abusing the examiner relationship.
Regarding interest charges, we looked into that several years ago, and decided it was too much work.  I just try to make sure I only work for clients that pay timely.  Those that do fall behind, we stop working for and are happy when we collect our original fees.
I don't know for sure, but I would think to collect interest fees, you would need a bilateral contract with the client, at least here in Ohio.
Not something I have had to do for years,  but you can always check the records to see if the deal has closed, and who filed it.  Doug