The UPU put the onus on the country of origin to make sure the rules were complied with, not the recipient country.
One gray area - it is the senders responsibility to enter the harmonized tariff code on the paperwork and the UPU rules state that the absence of a number will likely cause delays or returns.
Most countries wouldn't care squat over a chip in an envelope but a few (Belgium, Germany, Denmark spring to mind) have been a$$holes over the past few months.
I actually have 3 returns here where everything was filled out properly - presumably the recipient refused to pay the duty - in all cases the customers never bothered to contact me.