Sorry that you've been hit with a dispute! You should submit as detailed evidence as possible, indicating that your pricing is seasonal, and that this was visible to your customer at the time of purchase.
The fine print really does matter when it comes to disputes, unfortunately. We go into some more detail about this in our docs (https://stripe.com/docs/disputes#what-to-submit …). Please feel free to send us some detail about the charge via https://support.stripe.com/contact and we'll be happy to take a look.