Got a CF-28

The thing I would get straight before you write anything: who is your seller?

You bought from the trading company. Under 19 CFR 152.102(f) the price actually paid or payable is the total payment made by the buyer to, or for the benefit of, the seller. That is your price to the trading company. What the trading company paid the factory is a different sale, and it is only your problem if you are
claiming first sale. You are not. So the document you cannot get, the one showing the factory price, is not a hole in your declaration. It is a hole in a claim you never made.

That reframes the whole response. CBP asked for the purchase agreement with the supplier, and your supplier is the trading company. Send the pro forma, the wire confirmations, and whatever terms the trading company gave you. Wires tied to specific invoices are proof of payment. That is what the 28 is asking for.

The part I would go check tonight is the freight, because that is where your risk actually is.

152.102(f) puts international transportation, insurance and related services outside the price actually paid or payable, and 152.103(a)(1) Example 4 works it through: the ocean freight and insurance charge is excluded because transaction value excludes C.I.F. charges.

But the carve out only reaches an amount you can identify. Pull the three 7501s and see what you entered. If the entered value equals the lump sum, you deducted nothing, you have nothing to defend, and your answer is short. If it is lower, something gave your entry writer a freight number and you need to find what. If nothing supports it, the conservative position is that the whole lump sum is the price actually paid or payable, and the fix is a tender, not an argument.

On the cover letter, send it. The instinct not to overexplain is right in plenty of places but a 28 is not one of them. A short factual letter saying here is thesale, here is the payment, here is why no separate factory agreement exists, is the thing that closes the file.

If next week is genuinely too tight, ask for more time before the date rather than after. Extensions get granted routinely when you ask early and say why.

Last thing so a 29 feels smaller than it does right now. It is a notice of proposed action, not a bill. 19 CFR 152.2 is the section, it triggers when the estimated increase in duties runs over $15, and liquidation holds at least 20 days from the mailing, so you get a window to respond. Getting one is not losing.