The customer is not always right, but they are always the customer.
Sure, people are going to take advantage of you. But overall, you still shouldn't make your policies too strict because a few people will take advantage of you.
Treat the customer like they matter to you.
Mr. Marriott told me, "Always give the customer the benefit of the doubt." Go in favor of the customer.
If you find a trend or something, in particular, getting abused and it becomes a problem, then you can make a policy dealing with it.
If a request is unreasonable, it may be time for the manager to get involved. But you will be better off given front line staff the ability to deal with customer requests.
Customers are more likely to continue to do business with you if you treat them with respect and make it easy to do business with you.
You will have to continue to find ways to get better faster. And find ways to serve customers in new ways.