Subscriptions paid with whole season and Single payment SKUs may have remaining paid credits when a season completes (ends.) This means the member paid for more box credits than they received.
This probably means they placed box holds and did not double up boxes. The season can end with credits on account, there are no automatic processes to refund nor transfer value to the member's cash account.
Most farms simply ignore the unused credits. If you wish to add to the member's cash account for the value of the unused boxes, go ahead. Sometimes farmers do this for a lesser value, i.e. 'charge' for a handling fee.
Be clear about your policy for unused boxes in your FAQs and/or Policies to prevent post-season member requests.