The shipping method applied to the order should be identified in your Sale Notification email. First Class would only be applied for shipments under 13 oz. and should be quite obviously cheaper than you'd expect for Priority. You can also choose to configure your Shipping Rule(s) to use only Priority and/or other USPS services such as Media Mail or Parcel Post, rather than the USPS Cheapest method that only compares First Class vs. Priority. If you set up Rules for more than one method, the buyer would be shown a choice of rates/methods in their Cart before checkout.
Tax, Shipping and item quantity are set in the E-junkie shopping cart before checkout, so make sure you are using the standard E-junkie Cart button codes we provide in your Seller Admin. Buy Now buttons bypass the cart and take buyers directly to instant checkout for just one item at a time, so in that case there is no chance to change quantity or calculate tax/shipping in the cart before checkout. Also, make sure you enable Sales Tax, Shipping/Buyer's Address, and Let Buyer Edit Quantity in the product's settings, and on the product's Shipping Options screen, set Packaging Capacity to 20. You may also want to specify an empty weight for your product's chosen Packaging Type, in Seller Admin > Cart Shipping Settings.
This help page explains our Shipping calculation settings in more detail:
This help page explains our Sales Tax calculation settings: