Flat Rate shipping is most commonly used if all your items are similar in size and weight. If enabled it will apply to all your items. In most instances you should not enable USPS or UPS if Flat Rate is enabled. If you do, you will be required to add dimensions/weight for each item and buyers will be given the option to choose those shippers or Flat Rate when they reach checkout.
You can enable Free shipping on a per item basis and it can be extended to US only, US and CAN only or US, CAN and International. If you want to apply flat rate to individual items (rather than all items), turn Flat Rate off in shop settings and use the Free Shipping option per item, adding the shipping amount to the item price.