If you use automatic fixes regularly, you will want more credits than your plan gives you.
Buying a pack
Packs come in several sizes, with the larger ones better value per credit. Choose one in the credits area and pay with your card. Credits are added as soon as the payment succeeds, valid for 365 days from that moment.
Purchases appear on their own invoice, separate from your subscription, which is usually what a finance team wants.
Automatic top ups
Instead of watching your balance, set it to refill itself. You choose a threshold and a pack. When your balance falls below the threshold, the pack is bought automatically with your saved card.
You are notified each time one fires, so it is never silent.
Choosing a threshold
Set it high enough that the top up happens before you run out, and not so high that you are buying credits you will not use within the year. Roughly a fortnight of your normal use is a reasonable starting point, and you can adjust it once you see your own pattern.
If a top up fails
Usually the card. You are notified, and the top up will be attempted again once the payment method works. It is the same fix as any failed payment: update the card in Billing. See When a payment fails.
Turning it off
Disable it at any time from the same place. Your existing balance is unaffected.
Keeping an eye on it
Every automatic purchase is listed in your credit history, and there is a log of top up activity including anything that failed. Worth a look if your balance has moved in a way you did not expect.