Look man, you do you, we all will have our own judgment by Allah, so I'm no one to judge anyone, with that being said, it is asked from us to do what is right, regardless of what others do. If someone else is doing corruption, his judgment will be his own to face, as ours will be ours to face.
I see a LOT of us Pakistanis give billions in Sadqa/Zakat/Khairat and we do Hujj/Umrahs but we do these small "cost saving measures" that is actually corruption and stealing.
That is the IMTIHAAN for which we are here on earth, will we be honest even if we have little. May Allah guide us all and keep us in the right path, ameen.
And yes, asking seller to write lesser value does not affect shipping or handling. It will reach you safely, insha Allah. But if they write the exact model number then customs will find it's actual value through their CIF system. Sellers generally write a general category of the item if they are going to reduce the value.