Who Regulates Mail Order Pharmacies?

I have some complaints about Express-Scripts, a large mail order pharmacy.  I reside in California.  I mail the prescriptions to Arizona, but apparently the pharmacy filling my prescription is in Missouri.  I would also like to know if the prescriptions are always filled by the same pharmacy or in the same state--i.e., is there a single pharmacy for each region, or do prescriptions get handled some other way?  My prescription was held to a new law in Missouri, and neither I nor my doctor had any clue thst there was a new requirement based on Missouri law until my prescription was delayed and then cancelled.  I was never notified in the interim that there was any problem in filling this prescription. Also, the prescription was written, mailed, and received by the Pharmacy in December, 2010.  The new law went into effect as of January, 2011, so should it even have applied?