Tax differentiation usually takes the form of a broadly revenue-neutral package of tax changes, with increases in some tax rates and reductions in others, so the overall effect on revenue is rather small. A charge is levied when the product is sold and then fully or partly refunded when the good or its container are returned after use. Such arrangements can be mandatory or instituted on an entirely voluntary basis by producers themselves, where the recovery of items is sufficiently valuable (e.g. reusable printer cartridges).