Yes. And when you send the item back you will have the original purchase receipt and proof of payment of the duty on original import. The shipment must be posted as a tracked shipment that will be returning for the purposes of customs.

Again, often you can get away with it, but the above is how it is supposed to be sent, and of course you would have all the paperwork to support your claims

Doesnt matter. If it has value it is liable for tax ! If there is no value indicated and a dispute is raised, good chance customs will simply hold the goods until it is resolved. This can take months.
But there is a good chance the stuff will get through.

The bottom line is, there are many ways to try and reduce what you pay. Many of them work most of the time. But you have to bare in mind there is always a risk when doing so.
90% of the time it is worth it and things will go smoothly.

I was very lucky one time, I bought a load of stuff and it was sent via UPS. Lucky for me the driver dropped it off without realising there was duty to be paid. When I got the bill, the idiot had calculated the duty totally wrong. On the written value as if it was ££ instead of $$.

This dispute about duty owed carried on for 6 months. Letters, phone calls etc. If the driver hadnt dropped it off by mistake, I would never have seen my parts. And it was all down to their sheer stupidity and inability to perform simple reading and maths tasks. I was up front at all times and told them I'm quite happy to pay, when the correct value is calculated. Even at 6 months they still couldnt manage it !! The last attempt I seen, they had written everything out right right til the very end. Then for some unknown reason, they simply added the total to itself doubling it !!

After all that and another argument down the phone it all seemed to disappear into oblivion. So ended up paying them nothing lol

Ive had a few things with duty calculated more, and some less. Unless it's extreme Ive just ended up paying as it's less hassle.

And duty is usually calculated on the declared value of the goods. Worth baring in mind when getting stuff shipped. Although not all senders are willing to work with this. Just make sure all paperwork sent with the items reflects this value. And dont rip the arse out of it and make sure the value can still be sensible.