A 100 mg screening sample can look perfect. A 1 kg repeat order is a different product in commercial terms: method validation, impurity control, solvent handling, pack format and the document set all change. Buyers who ask about this early get a quotation that holds; buyers who ask after scale-up get a change order.
The arithmetic that drives the price
Peptide chains are built one coupling step at a time, and stepwise yields multiply. If a repeated step runs at 80%, ten repetitions give 0.8^10, about 10.7% overall; at 95% about 59.9%; at 99% about 90.4%. These are illustrative numbers, not our process data - but they explain why a longer sequence is disproportionately more expensive, and why a small method improvement at the screening stage matters at kilogram scale.
Scale it once more: to deliver 1 kg of final peptide at 10.7% overall yield you would need roughly 9.3 kg of starting material at that step, versus about 1.1 kg at 90.4%. Nothing about the quotation looks mysterious once the yield assumption is visible.
Three questions to settle at the sample stage
1) Which impurities are controlled, and not merely reported? A chromatogram can list twenty peaks; the specification should say which ones carry limits.
2) What purity window is acceptable batch to batch? A single "at least 98%" line makes the lower bound clear but says nothing about how much variation the process is allowed.
3) Which analytical method decides a pass or fail, and will both labs use the same column, gradient and integration rules?
What changes in the document set
At the screening stage, a COA and an MS trace are usually enough. A repeat supply agreement should also lock the specification, define what counts as a process change, set a change-notification period, and list which reports accompany every batch: COA, MSDS, TDS, HPLC / MS reports, and heavy-metal or microbiological results where the destination market requires them.
Third-party verification is normally requested once during supplier qualification, not with every shipment. Asking for it per batch is possible, but it belongs in the price discussion rather than as a surprise at goods receipt.
Packaging is part of the specification too
A 1 g sample in amber glass and a 1 kg lot in foil pouches do not behave identically through shipping. We ask about destination climate, expected storage time and whether the lot will be split on arrival. The pack format goes into the quotation because it affects stability data, and because the label has to carry the same lot number as the COA.
What we need before quoting kilogram scale
Sequence or product name, target quantity and expected annual volume, destination market, purity window and the documents your QA file requires. If a route cannot meet those limits reproducibly, we would rather say so while the sample is still on the bench.
The short version
A milligram sample proves chemistry. A kilogram order proves a process. Ask for the method, the impurity limits and the change-notification terms at the sample stage - that is when they are cheapest to agree.