Missing invoice is one of the common reasons why a parcel is being held in customs. Though it was retained, it doesn’t mean you’ll pay duty. Just set your expectation that you might pay. Give customs enough time to examine the package. Most probably, they put it on hold to figure out the actual price of the item. After acquiring enough information, they will evaluate if they should charge tax in your package or not. If they open your package to check the item inside or search for a receipt, they should tag your package as opened or it depends upon their procedure. If you think it’s taking too long, contact them immediately. If they ask a copy of the receipt, then send them a copy.
If the seller or merchant lowered down the total declared value of the item because you requested it, you may ask for a copy of pretend invoice. Take note, this is illegal. Doing this means you are breaking the law. You might get in trouble if you show them with a forged receipt and they have a reference of the real market value of the item. You should come up with good reasoning. Though you have a receipt, the total dutiable value might increase. Remember, a customs examiner has a way to check the real market value of an item and appraise it. Whatever the total fee that is showing in the customs form is final. You are obliged to pay it.
Now, do you need to ask the seller to put an invoice in the package every time you shop online? It depends. If you know your country’s customs requires a receipt then request it. This way customs will process clearance of your package faster.
Avoid requesting to lower down the item value. If the package was lost, the seller will not be compensated with the correct amount. Always remember that as a buyer it is your responsibility to pay any applicable tax that might be imposed in your package. It’s the law.