The most likely scenario for receiving an unexpected charge to your account is because you or someone on your team added a new team member or collaborator to your account. With most plans, doing this will incur a user charge for the month.
To avoid these charges from happening if this was not intended or you just wanted to share your assets with another person, use the Review Link feature instead. This is 100% free, unlimited, and the best method for sharing a quick link to a person for the purposes of leaving feedback.
Removing the user from your account will immediately credit the unused time back. The credit can be used for the next billing cycle charge or adding more users/storage to your account. Contact Support if you need additional help with this process.