Bing Ads has a bit complicated procedure regarding the accounts and they are in a process of refinement. They have something called as a prepay account where you add funds to your account and they deduct click charges as they occur.
You can add funds at any time. Available funds will be visible on the Account List and Account Details pages.
For prepay, the “Balance” in the Account List indicates the funds remaining. When the prepaid balance is depleted the account will be automatically be place on pause.