Travelers must be aware of the customs policies in the country they are traveling to, and should declare items as needed.
Carrying a lot of items of the same category (example: a lot of smartphones) could exceed the allowances in certain countries and cause you to pay customs.
Both parties (Traveler and Shopper) need to negotiate how customs will be paid (if any), either by leaving the item at the Airport Customs (where the Shopper can pay and receive his item) or arranging for the Shopper to reimburse the Traveler upon delivery (provided the Traveler has the customs receipt or other proof).
Travelers should be transparent with Shoppers about the other deals they're accepting. If a Traveler accepts multiple packages of the same category without informing all Shoppers involved then ends up paying customs, then HitchHiker has the right to not pay the Traveler Reward (partially or entirely) in order to reimburse the Shoppers.