carefull bro...i do a lot of internet trading, so be carefull...The best advice is
a) ALWAYS and ALWAYS and again ALWAYS send your product upon received of full payment. There is no paypal fee for buyer, only to you. (unless I am wrong) so why he bother want to share ? the fee will be impose on you when you withdraw money from paypal (unless again if i am wrong)
b) Poslaju is good, DHL is reliable but expensive, actually any courier company would do
c) Now at 900, it is quite a lot of money. To under declare would help him save up in GST (i dont know how much is in Singapore) but by under declare, u will declare lower amount for insurance and in the event if mishap, you will lose a lot. Whether or not to insure your package is depend on how confident are you with the courier service ? i think pos laju insurance is quite reasonable, but do your math, would it make sense against the risk ?
my best advice....put up your offer in MWF first, u might get buyer from malaysia, and it would make life much easier, better still if you live within close proximity of your potential client, you can easily go for COD. Save a lot of money, and secure transaction...Otherwise, cost to deliver a watch to client with pos laju within peninsular would not be more than RM 20 (depending on weight) and will save you a lot...the saving can goes to insurance. I recently send 9 watches without insurance and i guess i am lucky, it arrived the next day with no harm. But i am playing with my luck. best to get it insured....