Hiya from a fellow Sarah, Sara!
Oh, Sara, I wish I could give you a direct answer to this question. While I can tell you is that Walt Disney World Resort has a zero-fee cancellation policy for a
vacation package, including accommodations in the
Disney Resorts Collection and
theme park tickets, canceled more than 30-days in advance of check-in; I could not tell you if Costco has any additional cancellation fees nor what their cancellation terms would be. Please note that cancellation terms and conditions are subject to change.
What I can tell you though, is that Costco's customer service is
almost as good as Disney's so I recommend that you reach out to them. I found I have the best luck with Costco by using the chat feature on their website or through an email.
Sara, if you are concerned about a last-minute cancellation, I would recommend that you purchase travel insurance, just in case.
I know that you will have a magical time, Sara. I wish you all the luck! And please, come back here if we can help you decide where to play, stay, or dine at Walt Disney World Resort!
We love to hEAR from you!
Sarah B.