I regularly see documents where the win themes focus on excellent customer service. When backed up with evidence, this can be a very persuasive argument which many customers like. If, however, the accompanying correspondence dwells on terms and conditions, exclusions and uses contractual language then the customer can get confused.
It’s important your whole message is convincing so whatever style you choose, make sure it’s consistent throughout.
Is Your Tender Schizophrenic?
Consider the customer’s reaction if your main tender document is friendly, collaborative and helpful but your cover letter is formal, contractual and inflexible.