@rmeihls , assuming that you are US citizen/Resident(GreenCard)/Resident for Tax purposes, working as a self-employed does not care whether your "contractee/customer" is here in the USA or abroad and whether they pay you a fixed or invoiced amount or how many times a year. I am assuming that this client/customer has no "situ"-- office -- in the USA.
Ideally you should invoice, get paid and report your income on Schedule- C --- gross income, required/usual / alloable expenses. You will also be charged a 15.3% on net income for Self-Employment Taxes ( SECA-- equivalent of FICA for wage earners).
Please consider reading up material at this site ---> https://www.irs.gov/businesses/small-businesses-self-employed/self-employed-individuals-tax-center
Is there more I can do for you?
