Assuming you are located in the US, then you would simply enable Shipping/Buyer's Address in the product's settings, then on the next screen you can provide a shipping weight, select a package type and specify how many units of that product would fit in that package. As always, click Next until you can click Submit to save changes.
Going into Seller Admin > Cart Shipping Settings, you can specify your own location and also give an empty weight for that package type, select which countries you are willing to ship to, set a Handling charge, etc. When you click Next, that takes you to the Shipping Rules screen. If you will only ship to the US and UK (which you should have selected on the previous screen), you only need to add a rule for Country: United States and another for Country: United Kingdom, both using whatever USPS rate calculator you wish (Priority Mail is a pretty good bet for affordable cost with buyer-satisfying speed).