We don't have Costco membership, but 8.5% of $10,000 would be worth 2 years of membership (5 of us going on a 7 day Fantasy in July).
But, if we transfer it to Costco after booking later this month, can we still use Disney gift cards to pay off the balance in spring 2019? BJs has 4% off gift cards. 4% of $8000 balance would be another $320.