In the USA customs on phones and similar mobile computing devices is 0%. Unless they come from Japan and have a 16 bit or larger CPU, then its something outrageous like 200% which I assume is some ancient 1980s calculator tariff thats still hanging around. If you do not require a formal entry, typically there are no customs duties or charges at all. Formal entry is not required for packages sent by mail carrier with a value $1500, unless they are in a special category like munitions, produce, etc, etc. Even with a formal entry this category doesn't have any duty but your customs agent will typically charge a % for handling the paperwork. DHL used to charge me 0.25% I think, its been more than I year though so I can't remember exactly. This isn't true for most western countries, the EU has VAT and local sales tax, Canada has a national sales tax, many other countries have crippling duties.
In short, drop shipping to the USA is not a problem and won't add duties. I used to do a formal entries of this kind of merchandise about once a month when I had this going www.pengpod.com .