1. This is a new law and there will be a testing time, just like a new software being tested.
2. USCIS has clearly said that they will not accept any applications before or May 26. If the delivery carrier delivers it before the May 26, the case will be rejected.
3. Mailing the package before May 26, might make the USCIS think that it was not received on May 26, leading to a rejection.
4. The H4 EAD unlike the H1B has no deadline, therefore, you can file long after May 26 because there are no quota. Do not believe false rumors on the situation.
5. The USCIS is going to get an influx of cases on May 26 and as such they might just put all cases coming for this week to be received and processed on the same date as May 26.
For the above reasons, we are recommending not to rush and mail your package on May 26 or after not before. As they say only "fools rush in!"
We wish you all the best for your H4 EAD application, and should you need help please contact us at 510 742 5887.