I know that you can declare goods as samples. For example if someone wanted to buy a bulk of trousers you could send one declared as a sample. However there is usually a limit regarding the worth of the goods.
The other way is probably to send the goods with a "pro forma invoice" and declare them and then get a refund when they are returned. However this usually involves a bit of paper work and if you make a mistake with the paper work this could fail. If you are not familiar with customs regulations you should contact a customs broker and let them handle the paperwork.