Why do I need a customs stamp? /

If you export items via airplane, car or train yourself, you need to get a stamp from customs to prove that you have exported the items to a country outside the EU. You have to export your items and get a stamp from customs before the end of the 3rd month, after the month of purchase. 
Please check the full list of requirements here.

Were your goods exported by a shipping company, via ocean freighter or via airmail (DHL, UPS, FedEx etc)? Then you don't need to get a customs stamp. Check the requirements for ocean here. Check the requirements for airmail here.