The Balance will always reflect the amount of money in Euros, not the number of testing credits.
For Prepaid, the Balance is always above 0 and the testing is suspended when the Balance reaches 0. A new top-up is required in order to continue testing.
For Postpaid, it’s normal for the Balance to go below 0 based on usage. The invoice for Postpaid will always be issued in the first week of the month for the previous month’s usage. After the invoice is paid, the amount paid according to the invoice is added as plus to your account - which means that the remaining balance reflects only the tests issued in the current month that haven’t yet been paid for and/or invoiced.