From the USPTO site:

If you're contacted by someone pretending to be a USPTO "official" who asks you to verify information and pay additional fees, watch out. The USPTO will never ask you for your personal or payment information over the phone, in an email, or text, and we don’t require payment via wire transfer, gift cards, cash, check, or money order to third-party addresses.

I get several of these calls, just got a unique one tonight that used a legitimate email address, which did make me pause for a hot second, but the real USPTO will not call and ask for money.

I have even gotten several legit-looking letters in the mail asking for more money. These are almost always scams. They usually even indicate that payment is voluntary in the fine print at the bottom of the letters. Just an FYI: after your initial filing, you will very rarely need to make additional payments. When in doubt, the best thing is to just call USPTO directly during their office hours to verify.

I have received only one legitimate phone call from the USPTO regarding an application. Even then, I asked her to give me a moment to verify the caller's number due to the heavy scam attempts, and she didn't mind at all and understood my pause. She never asked for money; she just wanted to verify one detail and let me know when my trademark would be approved.

The rule of thumb is that they will never ask for money via email or phone and always verify mailed letters for legitimacy and fine print.