Saying no without losing the customer

Say it in the first line

Burying a refusal in the third paragraph reads as evasion and guarantees another exchange. State the position, then explain.

Give the reason once

One clear sentence. Over-explaining sounds defensive and invites negotiation of each individual point.

Offer the nearest possible yes

What you can do, what it would cost, or when the position might change. A refusal with an alternative is a service. A refusal alone is a wall, and walls get escalated.

Do not hide behind policy

“It is our policy” is heard as “I have stopped thinking about you”. Give the actual reason the policy exists, or if you cannot, say plainly that this is a decision you are not able to change.