Very shortly after your order is placed, it is assigned a tracking number. If you have a tracking number that is not showing movement, or starts and stops showing movement, it is likely your order was shipped using what is called last-mile delivery. This means your order was shipped using DHL, FedEx Ground Economy, UPS Mail Innovations, or any number of other carriers, to your local USPS sorting facility. Your order will be delivered by USPS.
Try searching the tracking number on DHL eCommerce, FedEx, UPS, 17Track (https://www.17track.net/en), or Sendle for tracking starting with "S" (https://track.sendle.com/). The package may be in transit, but has not yet been received at your local USPS facility.