Frankly speaking, even China Post itself can not control the speed of delivery outside China. International Postal System is a loose partnership network financially sponsored by governments of many countries.

In your case, the parcel will be sent to Russia Post system by airline , then transit from Russia to Estonia by Russia Post , and finally delivered by Estonia local post office. Every partner can cause the delay and none will be punished for the delay.

You can see the post airmail is a cheap price service, speed is slow but somewhat acceptable. If accurate and fast service is required, you probably have to use private courier such as DHL,UPS etc. Otherwise just be patient.