none is right -- not replying until after May 15th doesn't hurt AMCAS in the least -- it only affects you, the schools, and your fellow classmates.
The May 15th guideline seems to apply to all schools across the board, regardless of participation in AMCAS, dual-degree programs, etc. However, I have heard of some MD/PhD programs that wanted decisions by April 15th. If that was the case, obviously you should be aware of that now.
If you are concerned about a letter of withdrawal not arriving at the admissions office until after May 15th, I would go ahead and send your letter by regular mail, but also call or email the admissions office as well. That way they will know to expect it and can start making plans, and you don't have to worry about receiving threatening letters. No need to overnight or FedEx anything.