A dated copy of the official page and FAQ as read, a written summary of the fee, gift and family terms on that date, the pending petition's receipt and priority date, and every question put to the official channel with the answer received.
Build the file before the payment, not after
Because the framework is administrative, the terms in force on the day a decision is made are the terms that matter, and they may not be the terms in force six months later. A file assembled in advance protects the applicant twice: it records what he relied on, and it forces the questions that ought to be asked before money moves. The core of it is a printed or saved copy of the official page and its FAQ with the date of access, and a one-page summary in his own words of the processing fee, the gift, what is refundable and what is not, and who may be included as family.
Beside it belong the pending petition's receipt notice and priority date, since that date is an asset. Any correspondence with the official channel should be written rather than telephoned, and the reply kept. If he later pays, the receipt goes into the same file with the version of the terms it was made under.