Volkswagen Golf GTI 2016 Owner's Manual page 257

Never use more than 1 extender with a safety belt. Using more
than 1 extender can change the way the safety belt passes
over the body and can cause serious injury.
Never use the safety belt extender to secure a child restraint.
Never use a safety belt extender on your Volkswagen that you
got from another automobile manufacturer or from an
automotive parts store.
Never use the safety belt extender you got for your vehicle for
any other vehicle, regardless of make, model, or model year.