My goal is to provide an EXCELLENT service.
The bonus is optional - but highly appreciated.
If you are self-employed - and your self-employment income is from sources outside the US - it is not taxable for nonresident.
But if that is income from US sources - it is subject of both - income and self-employment taxes.
There are some complex rules to determine the source of income. You may take a look - page 12 IRS publication 519 - http://www.irs.gov/pub/irs-pdf/p519.pdf - Table 2-1. Summary of Source Rules for Income of Nonresident Aliens.