If you are shipping an item you have bought online back to the seller for repair, you must register the item as temporarily exported before it is sent out of the country. If you fail to do this, you run the risk of paying taxes twice.
Procede like this
a) Before you ship the goods out of Norway, you register (declare) the export yourself with Norwegian Customs. To register the export yourself, consult your local customs office. Overview of customs offices
b) You can ask the shipping agent, in most cases Posten, to do it for you. If you choose to let the shipping agent do it for you, you have to state that the item should be registered as a temporary export for repair.
2) The product must be re-imported within one year after exportation.
3) Re-importation must be done by the same who carried the goods abroad
4) There neither is nor will be applied reimbursement of customs duties when exporting the product.
Taxes when the item is returned
Warranty matter: you do not have to pay tax when the item is returned if the repair is a warranty matter that you should not pay for.
Repairs: If you pay for the repairs yourself you have to pay VAT and possibly customs duties of repair costs and shipping back and forth.
If you have not followed this procedure, and tax and customs duties have been miscalculated, you may apply for a refund here.