Based on your query, I would recommend using Grammarly to help you with writing business letters. This tool is designed to assist with writing tasks by providing real-time feedback on grammar, punctuation, and spelling errors. It also offers stylistic and best-practice writing suggestions, which can be particularly useful when crafting professional business letters.

The key features of Grammarly that could be beneficial for your needs include:

• Grammar and Spell Checking: This feature will ensure your business letters are free from grammatical and spelling errors, enhancing their professionalism.
• Stylistic and Best-Practice Writing Suggestions: These suggestions can help you improve the tone and style of your business letters, making them more effective and engaging.
• Real-Time Comments and Suggestions: As you write, Grammarly provides suggestions and explanations, aiding in the writing process and helping you to improve your writing skills over time.

In addition, Grammarly offers a color-coding process to differentiate errors and provide immediate feedback, which can be very helpful in quickly identifying and correcting mistakes.

Please note that while Grammarly does offer a free version, the premium version, which includes additional features, is available for $12 per month when billed annually. This might be a worthwhile investment if you frequently write business letters and other professional documents.