It’ll automatically use your credit.

After registering, verifying your email, adding a credit card you will receive your credit, you can find it in your billing page

I just signed up and created my first droplet. and went to billing and it says my balance is $0.00 after using a $10 referral link. I want to try out deployment strategy first with a single Ubuntu box before going to production. should i wait till the credit is applied or has the credit already been applied to the account

$25 Digital Ocean Credit 2018

Use this for $10 credit
For $15 more credit, use Promo Code LOWENDBOX.

I have the same problem, my credit is more than 15$ but digitalocean used my paypal account for the invoice.