If you paid for product(s) with free delivery, there is no defensible way they can "add" imaginary value to the shipment.
Yes, there is.
The value of the shipment is the value of the goods plus the value of the shipping (in some countries).
But more importantly, the value is usually defined something like "the fair market value"
Therefore, if you didn't pay the shipping (free shipping) then you got a discount, but that doesn't effect the value of the shipment.
They do it this way because they need to be able to inspect the goods and check if the value is legit.
If for example you buy something with a 70% discount (and only paid 30% of the value of the item) the customs value of the shipment is still the full value not the discounted value.
If the customs value was the value you paid then it would incentives fake invoices showing a discount to reduce import fees and import customs dept wouldn't be able to know by looking at the item if it was a totally legit discount or a fake discount. So they have the rule that the customs value of a shipped item is the fair market value of the item, not the discounted value. It makes it simple to check if the declared value makes sense just by looking at an item and searching to see how much its normally worth.
So, the real question is, is free shipping a discount or did you just pay for the shipping a different way, as part of the value of the items.
In which case you didn't get a discount and the fair market value of the item is what you paid and nothing else should be added.