Personally I think this is tricky one and a mixed bag .....
I would never ask for an item to be undervalued , but if a seller offered I might take them up on it .
If someone asked me to undervalue an item I would refuse , but if I agreed to do something I would honour the agreement .
I know that customs charges to my country (aust) are nil or minimal , but I know what things are like for most places and would be resigned to expecting any 'surprises' customs threw at me .
If I was unsure , as a buyer or seller , of reliable expected courier charges I would double check , and organize an alternative should the courier prices seem excessive.
To your credit you have been understanding on most of this and give Tai some credit (in amongst the frustrated retelling , and partial complaining) in his dealing with the process , and praise the bat highly . You are in your rights to be disappointed, but other than he forget to include a scuff sheet (which was rectified) and not recieving a bat cover , Tai has not really done anything ( or , arguably , much ) 'wrong' , although perhaps he could have enquired with you/dhl/other couriers further to find the cheapest option ? , and either he forgot to undervalue the bat ,as agreed upon , or he changed his mind and didn't inform you (either possibility is still disappointing after an agreement has been made, and this is the part I'd be most frustrated with if this is what happened and it happened to me), or perhaps it's also possible there was some misunderstanding and Tai never knew this agreement was what you were asking for ( I know you are saying this isn't possible /likely but I'm just trying to allow for all possibilities here).
To Tai's credit he compromised profit numerous times to provide extra service/absorb the additional costs where possible , and he gave you options including a refund . He also later sent the scuff sheet .....oh , and he made you a cracking g1 bat for 250 quid .
I suppose the moral of the story is : dont use dhl for bat delivery to the u.a.e .