How to protect electronics from static electricity during shipping
Silvery anti-static bags are (should be) conductive, so they act as a Faraday's cage against ESD coming from outside the bag, i.e. they protect from the ESD events coming from outside the bag. If the electronics in the bag rubs against something inside the bag that can build-up static charges, the bag won't protect it. 

Therefore you should ensure that whatever is inside the bag, especially if it could move and rub against the electronics, doesn't build up charges. For example, the foam you use, if it is in contact with the electronics inside the silvery bag, should either be somewhat conductive or static-dissipative, i.e. a material that doesn't allow charge to build up (that pink plastic that is sometimes used to wrap electronic circuitry and components is usually a static-dissipative material). 

Dave Jones (Author of EEVblog) has made a couple of videos about anti-static stuff that you may find relevant: EEVblog #3, EEVblog #247 and EEVblog #250.

Regarding your 3rd point, probably you cannot do anything to protect your circuit. The circuit must self-protect itself, i.e. it should have enough protection circuitry built-in. This is because customs officers could do pretty anything to the thing they are inspecting, short of breaking them mechanically (they even could do that in some countries, if they suspect the thing may contain dangerous or illegal materials).

If you are unlucky enough, officers will reach for the circuit board and will examine it thoroughly holding it in their hands and handling it brashly (if they don't think it's dangerous, they won't lose time with careful handling). In a dry day and with the right combination of statics-generating surfaces your circuit could be well zapped to death, without suitable ESD protection built-in.

EDIT (prompted by a comment)

Just a simple example: you mention wires coming out of the enclosure. If those wires are directly connected to inputs or outputs of chips/components that are ESD-sensitive, those connection must be protected by adding ESD-protection circuitry on the board. A simple protection measure is to put backwards connected diodes from the pin to be protected to the power rails, maybe adding a current-limiting resistor in series. Something like this:



There are more advanced strategies as well. There are even dedicated ICs whose only purpose is to protect the I/O pins of other chips/boards. You may be interested in these documents:


Texas Instruments Guide to ESD/EMI protection chips
ESD protection for I/O ports (MaximIntegrated application note)
Design considerations for system-level ESD circuit protection (Texas Instruments Article)