Texas landlord/tenant code specifically addresses late fees as follows (note that your city may have supplemental laws as well):
Sec. 92.019. LATE PAYMENT OF RENT; FEES. (a) A landlord may not charge a tenant a late fee for failing to pay rent unless:
(1) notice of the fee is included in a written lease;
(2) the fee is a reasonable estimate of uncertain damages to the landlord that are incapable of precise calculation and result from late payment of rent; and
(3) the rent has remained unpaid one full day after the date the rent was originally due.
(b) A late fee under this section may include an initial fee and a daily fee for each day the rent continues to remain unpaid.
(c) A landlord who violates this section is liable to the tenant for an amount equal to the sum of $100, three times the amount of the late fee charged in violation of this section, and the tenant's reasonable attorney's fees.
(d) A provision of a lease that purports to waive a right or exempt a party from a liability or duty under this section is void.
(e) This section relates only to a fee, charge, or other sum of money required to be paid under the lease if rent is not paid as provided by Subsection (a)(3), and does not affect the landlord's right to terminate the lease or take other action permitted by the lease or other law. Payment of the fee, charge, or other sum of money by a tenant does not waive the right or remedies provided by this section.
So, first look to your lease to determine whether you landlord is charging a late fee. If so, compare it to the statute to see if it is reasonable. If not, consult a local landlord/tenant attorney.
DISCLAIMER: Brandy A. Peeples is licensed to practice law in the State of Maryland. This answer is being provided for informational purposes only and the laws of your jurisdiction may differ. This answer based on general legal principles and is not intended for the purpose of providing specific legal advice or opinions. Under no circumstances does this answer constitute the establishment of an attorney-client relationship. For legal advice relating to your specific situation, I strongly urge you to consult with an attorney in your area. NO COMMUNICATIONS WITH ME ARE TO BE CONSTRUED AS ARISING FROM AN ATTORNEY-CLIENT RELATIONSHIP AND NO ATTORNEY-CLIENT RELATIONSHIP WILL BE ESTABLISHED WITH ME UNLESS I HAVE EXPRESSLY AGREED TO UNDERTAKE YOUR REPRESENTATION, WHICH INCLUDES THE EXECUTION OF A WRITTEN AGREEMENT OF RETAINER.