If it has been 7 days go into your purchase history and cancel the order, they are most likely not active on there anymore.

If you bought the dress off of Poshmark (i. E using PayPal) there is no guarantee you'll receive your item. Poshmark only gives refunds on in-app transactions, and doesn't recommend buying on other platforms. That's between you and the seller. If you DID buy the dress on the app itself, note that all items with the Poshmark shipping label are automatically tracked and insured. To see where your order is, see the item's details under your purchases.