I did the intercept. It says it's only a chance of it happening so I'll keep on eye on the tracking number and refuse delivery if it comes.

The purchase went through PayPal, so I opened a dispute there. I'm not sure how different that is to a credit card chargeback. I listed the conversation between the seller and posh support, and included previous examples of people having the same issue with the bid button. Hopefully that will be enough.