Ivy here from Shopify.
Are you still encountering this error when trying to close out your store? You mentioned that you had just paid for your last cycle, so it sounds like you are in a new 30 day billing cycle currently. This means that the invoice for this new cycle will have to paid in full before the store can be closed fully - in these cases, we may be apply a refund of this fee for you but it will have to be paid first in order to allow full store closure.
As I cannot access your account specific information here via the Community I cannot say that this is the case with 100% certainty, but this is a possibility. If you would like to have a Support Advisor access your account to verify this is definitely the case, we’d be happy to continue assisting you through live chat, email, or callback. Please visit our Support Portal here and log in to your account to create a support request.
Since this is the next step I've marked this reply as the solution for the benefit of others who may discover this topic. Thanks!