You will be billed for extra reference overage charges when the total amount of overage charges is equal to or greater than the cost of your subscription plan, or at the end of the billing cycle (month/year), whichever comes first.
For example, Eddy Fix has a Basic Monthly subscription plan, which costs $39.95. Eddy submits an unusually high volume of references this month (he landed a great copyediting gig), and used up his reference allowance very quickly. He continues to Edifix his references by paying extra reference overage charges at the set rate of $0.16 per reference. Before the end of the month, Eddy's overage charge balance totals $40.04, which is greater than the cost of his subscription. At that point, Eddy is prompted to pay his overage charge balance before he can continue submitting new jobs. Eddy processes 10 more references before the end of the month, accruing an overage charge balance of $1.60. When his subscription renews, Eddy is automatically charged for this remaining balance.
To view your overage charge information (including your paid and unpaid overage charge balances), click Account>My Subscription and look for Unpaid Overage Balance.