When you try to checkout to renew Custom Design or Domain Mapping, the error message "This blog is not subscribed to some of these products. Did you mean to purchase them?" may appear. This issue has been discussed at this WordPress.com forum.
This error will appear if "you may be the Admin of the site now but if you are not the original Admin of the site then know that's critical when it comes to renewing upgrades." However, I am the original administrator. I could resolve this problem by trying to complete my purchase at wordpress.com/purchases instead.