Can anyone help?

I have received my saliva secretor/non-secretor test kit.  I live in the UK and the instruction card  states that on the customs form I must write that the product is non animal tissue and for pathology test etc.  I received a pre-paid envelope to return my test to US but I am not sure what the customs form is?  Is this something you ask for at the post office?  Can I just post the envelope back without?  

Anyone from the UK who has done this test who can help?

Here in the US we get a customs form at the post office. I would imagine it is the same in the UK.