What extra mile can you go you gain a new customer?

It depends upon the customer and the situation you are in:

Evaluate it within the limits and moral jurisdiction and see If can you go that extra mile which customer wants you to go:

If it needs to satisfy the customer through product and its features I shall do that to the best of my capability

If customer wants to service him and ensure followup and after sales service I shall do that religiously.

If it needs to give him extra discount I shall review that and if viable and  shall fulfill that immediately.

If it needs to help him out with some service at personal level and that doesn't go against my conscience shall do that to my best.