Replying to Shipping/ Seller Problems

I bought line 1260s from a seller on here almost a month ago.
  He told me that they were shipped USPS three weeks ago last thursday.  Im  not sure how long shipping is supposed to take but a month seems a little excessive.  How should I go about dealing with the postal service and the seller?  Im leaning towards postal service error because it seems like this kid has a decent reputaiton, which is why im not including his name.  Just looking for a little advice.