[mothered]

when and where will the package weight be checked?

In terms of a local delivery and generally speaking, at the drop off/service point, at the carrier's depot and when It arrives at the company's Inwards goods/receiving area.

Isnt the dry ice already gone at this point and with that the whole point of it lost?

No.

You'd need to accurately calculate the sublimation time based on atmospheric conditions, and also storage & transit time.

wont the box be leaking fog like hell Biggrin making it kinda obvious for everyone handling whats inside?

No, It's not directly exposed to external conditions.