Been there ...The USPS under the international postal rules governing the distribution of postal mail is OBLIGATED to hold any poste restante/ general delivery mail for 4 weeks (or is it 30 days).

When I am in Canada I have all of my Amazon purchases sent to a PO in Minnesota and then when I have maybe 10 or 15 book packages, I drive across the border, go through the US Customs and Border control hurdles and get my packages, Then I cross the border back into Canada and leave the country on my merry way - travelling.

Amazon WILL NOT ship used books to countries I frequent.. So Whenever I come back to Canada/USA this method is a legal workaround... Cheap too because I do not pay Canada Customs collectible taxes when I show my exit airline ticket (to show my intent is to leave the country with the purchases) ...

Poste restante is VERY useful. I have used it in wide and varied places that are subject to international postal union rules - including Australia, Oman and Iran - as well as the USA. Seriously folks, it works for either 4 weeks or 30 days. If delayed, I call the postmaster on Skype and ask them to hold it for an extra week. They are always willing to oblige and ask me questions about my impressions of travel in THEIR country...