One way to demostarte assertiveness while interacting with a customer is to

Explanation:

When dealing with a customer, and you consider he/she is not rigth, you should be able to express your point of view in a respectful way but clearly  preserving your rights. That is assertiveness.

You must confront him/her in a constructively way: make it clear why the claim is not correct or fair, even how it affects the your or the company's right: you undersant him/her but he/she must understand you too.

You should prevent the situation from escalating to greater proportions but you should not give in to unfair demands that involve a loss for the company.

its simple as this, microeconomics is the study of economics at an individual, group or company level, macroeconomics on the other hand, is the study of a national economy as a whole.

hope it =^-^=