If you are purchasing insurance from Disney with a package (which I always do because my dad has health issues), the best way is to add it on at final payment (45 days prior to arrival). Your deposit is fully refundable up until that point-UNLESS you purchased the insurance at time of deposit in which case they will subtract the non-refundable insurance from your deposit. It's worth the $150 to me to know that if I need to cancel after final payment is made, all I will lose is the cost of the insurance.
One caution about outside vendor insurance-be sure to read the coverage since many of them do not cover pre-existing conditions unless the insurance is purchased at the time of booking.