You are completely wrong. Customs officials in most countries do not open packages, they use the customs sticker declared value. (Source: I actually receive international packages regularly. I've had one opened package.)
The courier/post office will generally validate that that sticker matches the value you told them for insurance purposes either way.
Well, clearly your custom officials are different than mine then. I've received thousands of international parcels(mostly from asia) and yes, parcels are definitely opened(I know this because many times I had to personally drive to the nearest customs office and they would bring out the stuff they took out of my parcels for inspection).
Of course you can choose to not believe me, but I was explicitly told that the "value" field is completely ignored especially on any post from China, because 99% of the time it's just nonsense and pretty much everything arrives as a "gift"(I once had a brand new Asus laptop worth about $800 that was declared with value of $15, contents: "electronic cables" and "gift" - the customs officers wanted to see receipts of all payments I made for it before releasing it).
>>The courier/post office will generally validate that that sticker matches the value you told them for insurance purposes either way.
Now this seems completely bogus. I've also sent thousands of packages and no one has ever "validated" if what I am sending is actually worth as much as I put on the declaration. As long as it wasn't illegal to send it in the post no one cared.
If your packages are really being opened as often as you say then you are likely on some sort of list. Having received "thousands" (!) of packages from Asia might have something to do with it - that is well into what most people would consider commercial quantities and they have possibly flagged you as potentially running some sort of unlicensed import operation.
I can confirm that in my country (Australia) packages are rarely opened and if they are, it's seldom for duty assessment but checking for contraband of whatever nature. Even if they do flag you for a duty check they don't need to open the package - they do have x-rays, you know. Try and declare a $10,000 drone as a package of t-shirts worth $5 and the officer doesn't need to open the box to reject your "estimate". That said, most sites of any repute worth upholding will print the real value on the declaration, non-negotiable.
The courier/post office will generally validate that that sticker matches the value you told them for insurance purposes either way.