If you don't have a USPS Customs Form Envelope, you can also place those extra customs form sheets in a regular envelope or a ziplock bag that you label with the phrase Customs Forms, and affix that envelope to the outside of your package.
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Customs will sometimes open packages to inspect the contents and ensure that they match what is listed in the declaration form.
What TSA wants is a bag that is the approximate dimension of a quart-size bag and is clear. That means that a ziploc bag from the grocery store works just fine as long as it is not larger than one quart.
For goods over the £1,500 threshold and restricted and excise goods, and goods over 1,000kg you will need to make a full customs declaration either by yourself or through an agent.