I received W-8BEN in the mail. I've been in the US for the past 5 years. I paid all my taxes on time. Does it mean I have to pay more? What do I do with this form?

This form is used by nonresident alien individuals to claim exemption from withholding on compensation for personal services because of an income tax treaty or the personal exemption amount.

Unless you are an international student or are exempt from the substantial presence test, you don't need to complete this form as it is for nonresidents.  You need to contact the company that is requesting this information and advise them that you are a resident alien and you would need to complete form W-9.