Beware! Scam artists are out there looking to prey upon unsuspecting patent and trademark holders with fraudulent requests for payment of fees. This should come as no surprise, since there are well-publicized scams of every variety including the infamous "Nigerian prince" variety. However, IP-related phishing can be particularly hard to detect because these fraudulent solicitations often look remarkably official. The U.S. Patent & Trademark Office (USPTO) has provided a helpful discussion of fraudulent solicitations pertaining to USPTO fees at https://www.uspto.gov/patents-getting-started/using-legal-services/scam-prevention/non-uspto-solicitations. The USPTO also recently warned of fraudulent emails that are being sent to owners of U.S. trademark applications and registrations -- these emails appear to originate from the USPTO domain (@uspto.gov) but are actually a scam.
The World Intellectual Property Office has also recognized the need for caution, warning that "requests are typically designed to look like legitimate ones from an official source, but are created by unscrupulous companies and individuals who are trying to defraud you." For more info, see https://www.wipo.int/finance/en/hoax_warning.html.
The takeaway from all this is simple -- if you receive a solicitation or "invoice" that you have any doubts about, please contact us.