To the extent that the person receiving the shipment has any influence on the selection of the winning bid, the Cover Letter presents another opportunity to sell your story and ‘why you’ arguments.
Use your cover letter to open with a customer benefit statement, not a cliché or seller focused opener. Cite your key discriminator, directly tied to a key prospect need.
Keep the cover letter short – a single page – and don’t let it expand to become another executive summary.
Have it signed by the most senior person they have interacted with in the Capture phase, not just the CEO (if not actually known to them)
Close with a guide to what is included and where they will find what they need in the proposal.