Ads on Spreaker must be pre-paid. You can buy "Credit" that will be used to pay for the advertising impressions your campaigns generate.
You can use your credit or debit card to buy this credit, and purchase more as you go along. As your Ad Campaigns run, they'll generate impressions (one impression is counted every time a banner is shown to a user), and the cost of these impressions will be deducted from your credit at the end of each day.
The current cost per impression is €0.002 (or €2.00 per 1,000 impressions). If, for example, your banners are seen 5,000 times in one day, you will have used €10.00 of your credit.