Each country has an import free threshold. This threshold varies per country. If your purchase amount exceeds this threshold, you will be required to declare your goods and pay import duties upon arrival in your country of destination. The thresholds of most countries can be found at customs-guidelines-for-travel.
Declaring goods is a responsibility each traveler has, whether the VAT is claimed back or not. Once you leave the EU, it is up to customs in your home country to check if travellers declare their goods.