“We are not sure our main supplier is charging us fairly.”
Concentrating spend with one supplier only pays if you get price certainty back. Your own purchase records will tell you whether you did.
A healthcare facility had given one supplier preferred status and suspected it was not getting the pricing that came with it. We reconstructed months of purchasing from raw transaction records and benchmarked the supplier against its own cheapest price rather than a market rate, so the findings could not be dismissed as market movement. They went into the renegotiation with a line-by-line list of every item they had overpaid for.