So, just to clarify, you did receive payment, right? I mean, I'm looking at the Posh Protect page, and she had 3 days from receipt to file a complaint. If it's been longer than that, and you were paid, she's SOL with her nonsense.
Otherwise, I'd contact Poshmark ASAP and find out what the policy is. I'm quite sure there's a policy for this - because EVERYBODY would be trying to do this, you know? Or at least bad people. Keep us posted!
ETA: As a longtime seller, you've long established your credibility. Don't respond to the buyer directly, keep all communication thru Posh. AND again, contact them ASAP!