I haven't submitted anything yet, still just collecting stuff and uploading. I'll submit what I have once I have it and we'll see what they say. I have a few 1099s from my US bank accounts and have uploaded those as proof of domicile.
I won't have a job or an apartment yet for domicile, so I'm just going to have to rely on other things -- US bank / credit card accounts, voting record, permanent mailing address in the US with active driver's license, and our temporary residence permits for the Czech Republic (implying that we don't intend to be here permanently).