The official program instructions current on that date, written identification of who receives each payment and under what conditions, a per-person breakdown for the household, and a statement of what happens to any payment if the application is refused or withdrawn.
Get the money terms in writing from the official source and the intermediary
Because the processing fee is described as nonrefundable and the gift is by definition not returned, the documents that matter most are the ones that fix who is paid what and when. Print or save the official page as it reads on the day, including the fee and gift figures and the family terms. Ask any intermediary for a written engagement that separates its own fees from government charges and the gift, names the payee and account for each, and states the conditions under which any of it is returned.
Hypothetical example: a Boischatel clinic owner is quoted a single all-in figure; she should refuse to proceed until it is split into government fee, gift, and consultant fee, because the official page describes those as distinct steps with distinct timing. Assemble the ordinary immigration documents too: passports, civil records for each family member, and any prior U.S. immigration paperwork, since vetting will require them.
Keep every receipt and every version of the official page you relied on.