I have no experience on the international insurance, but I'll give you my opinion in general. I always try to think about what I would have to go through to collect on the insurance. Is it you or the shipper that has to fill out all of the insurance info? How much of a hastle will it be and how much money are you going to get back?
In your case, lets ignore the effort it would take to collect on the insurance. if you take the insurance, you are definitely going to lose 20, but you eliminate the risk of losing 50. I say don't pay the 20 for insurance and the worse case is you lose 30 more than you would have lost if you took the insurance and hand to collect on it. Most likely nothing will happen to the shipment, and then you come out 20 ahead. Almost half the value of the order is a pretty steep price for insurance, do they loose half of the shippments :-)