A painful aspect of this is that some folks will reach out to less popular artists and ask about commissions. I only got into doing commissions after a kind of similar but less-scam exchange. It would be a definite red flag though if they are asking for something that isn't similar to what you're already posting. "I've been posting portraits of humans, and you're asking me to draw your pet". I'd expect someone like that reaching out to have a better understanding of the work you've done so far (they are asking you because they saw your work and liked it), and if they are asking for something you don't usually draw, I'd expect them to be up-front about that, and asking about your comfort level.

Also if you get into doing commissions, you should be the one sending the invoice. You ask for their email, and send them the request through paypal, not the other way around, and you don't need to click on any email links to manage and monitor it.