I have noticed that although I have two free credit vouchers added to my account, Google is debiting the credit that expires the latest. Plainly this is extremely unfortunate and not good practice.
Therefore you may well have credit expiring in a few days (or weeks) but unable to use it as the longer term credit is being debited instead of the shorter term voucher.
Not what I would expect from Google.
NOTE: On the plus side my request for “evaluation” TPU’s was promptly answered and given. Kudos there to the team!