You are probably entering the US on a tourist visa. In that case, you are not allowed to work. MSCs offering shops in the US will require a SSN or TIN, and with a tourist visa, you are not eligible to get either of those.
If that sounds unfair, US citizens visiting Great Britain, the EU, or most other countries on a tourist visa are not allowed to accept gainful employment (mystery shopping as a contractor is considered gainful employment) when traveling as a tourist either.
Shopping Southeast Pennsylvania, Delaware above the canal, and South Jersey since 2008