Tell them that you are the senior sourcing agent for your company and that your boss insists on testing 20 finished units with your audience before moving forward with mass production. This makes it clear that the decision is not up to you, but rather a higher authority, which can be more convincing.
You can say something like:
"Before we move forward with the mass production, my boss insists that he wants 20 finished units so that we can do some testing with our audience. Can you please quote me how much it would cost for you to create 20 units of this product, completed and finished in the packaging, and send it to our office?"
If they still decline, you can find a partially stripped down version of the product you want and get 20 units of this. That way the supplier may be more apt to provide it for you.
Also note that it's normal for the cost of these samples to be higher than the quote for mass production.