Generally, you should be sent a mail that states your package has been shipped and it should also give you an order number that you can use to track it via usps tracking.
If you saved the mail, you can just google usps tracking and copy/paste your tracking number and it will show you where your package is. You should also be able to view previous orders in the site store. Click on the link to the site store, and then go to orders. It gives you a list of your previous ones and what you bought.
If that is the case, you should contact either your local post office or if you live in an apartment/rent home you should contact the owners of that. If it says it has arrived, but it hasn't, the fault is likely with the shipping service.