Beware of misleading notices. Such notices may give the impression they are an invoice or represent your law firm or a governmental office. The US Patent and Trademark Office provides some examples of entities known to issue such notices to parties that have filed for trademark protection. https://www.uspto.gov/trademarks-getting-started/caution-misleading-notices
To provide just one example of how a company may mislead a trademark filer, the document is sent directly to the client instead of their attorney that filed the application. The document may appear to be an invoice, but contain fine print indicating that it is not invoice, but rather a contract to perform a specified service.