The dated official page, the intermediary's proposal in writing, its written answers on regulation and refunds, and a receipt for every payment identifying the payee and the official instruction it responds to. Verbal assurances are not evidence of anything.
Build a record that would explain every dollar later
The file serves two purposes: it lets the applicant compare what he was told with what the program says, and it protects him if a payment is later questioned. Keep the official page and FAQ printed with the date and time of each check, because the framework can change and the applicant needs to show what the terms were when he acted. Keep the intermediary's proposal, engagement letter and fee schedule, and its written responses to the questions about who regulates it, whether any deposit is refundable, and how its fee relates to the official processing fee.
Keep the eligibility analysis for the underlying immigrant classification, prepared before any payment. Then, for every payment, keep the official instruction that requested it, the confirmation of the transfer, and the receipt naming the payee. If a payment was made to the intermediary rather than under an official instruction, the record should say why.
With this file, any later question from a bank, a tax authority or an officer can be answered from paper.