[mothered]

in my experience most returns here in the uk take multiple days to get back to the warehouse.

^ This Is what I'm referring to with calculating dry Ice sublimation time.

Define "multiple days"?

so what complication would it cause if if the package were to be taken from the local post office to the depot?

As said, calculation of dry Ice storage time.

are packages weighed multiple times even after dropping it at the post office?

If they're taken to the depot, weights & dimensions are recorded to check In the consignment.

if so what complication can this cause, are you saying im better off dropping it directly to the depot?

I'm saying you'd need to research their procedures, thereby you can make an Informed decision with the amount of dry Ice needed.