No, I think you don't pay the 30 % witholding tax if you run Amazon FBA business from abroad, I educated myself and I read that (depending on your countires tax treaty) you need to have a permanent establishment in order to pay income tax in the us which is defined:
2. The term "permanent establishment" includes especially:
a) a place of management;
b) a branch;
c) an office;
d) a factory;
e) a workshop; and
f) a mine, an oil or gas well, a quarry, or any other place of extraction of natural
resources.
4. Notwithstanding the preceding provisions of this Article, the term︀ "permanent
establishment" shall be deemed not to include:
a) the use of facilities solely for︁ the purpose of storage, display or delivery of
goods or merchandise belonging to the enterprise;︂
b) the maintenance of a stock of goods or merchandise belonging to the enterprise
solely for the purpose of storage, display or delivery;
c) the maintenance of a stock of︃ goods or merchandise belonging to the enterprise
solely for the purpose of processing by another︄ enterprise;
d) the maintenance of a fixed place of business solely for the purpose of︅
purchasing goods or merchandise, or of collecting information, for the enterprise;
e) the maintenance of︆ a fixed place of business solely for the purpose of carrying
on, for the enterprise,︇ any other activity of a preparatory or auxiliary character;
f) the maintenance of a fixed︈ place of business solely for any combination of the
activities mentioned in subparagraphs a) through︉ e), provided that the overall activity
of the fixed place of business resulting from this︊ combination is of a preparatory or
auxiliary character.