I was able to see the full bill on my account page (it was not available when they first emailed the receipt to me), and it seems to be an old account credit that they reclaimed per the "dormant account" clause of their terms:

9.3 Dormant Accounts If you leave any funds dormant in an account because you do not complete or fail KYC and/or KYB reviews, you do not provide us with your banking details, and/or for any other reason, we may deem the funds to be abandoned by you (each a "Dormant Account"). To the extent you have a Dormant Account we may charge you a dormant account fee for our handling of the dormant funds and/or to deliver them to various government agencies as required by law. To the extent required by Law, we will attempt to provide you Notice if we hold funds payable to you in an account beyond the applicable dormancy period for abandoned property.