Not sure what you mean about extending hosting services.
You can change when you receive those thresholds alerts on your billing page.
If you add a credit card to your account, digital ocean can take money out automatically to make sure your account is topped up, if you use PayPal you will need to add money in advance yourself.
Digitalocean recommends that you always have a positive balance in your account if you cannot pay and you need more time to gather funds you can always create a ticket with digital ocean telling them this. It’s better telling them in advance that you will be late in paying rather than digital ocean automatically suspending your droplet because you couldn’t be bothered to tell them.