Draft Formal Complaint Letter

I need a concise, persuasive complaint letter addressed to an individual who is my client or customer. The goal is to lay out an administrative error that led to an incorrect benefit calculation, cite the exact dates and dollar amounts involved, and request swift correction... (Budget: €8 - €30 EUR, Jobs: Administrative Support, Business Writing, Content Writing, Editing, Legal Writing, Proofreading, Technical Writing, Writing)