If I publish a listing and one week later I upgrade it to Featured for 15 days, what happens to the balance of credits I spent when I originally published the listing?

In the scenario when you publish as Standard and upgrade, the original number of credits used to publish the Standard listing will be discounted from the price. This is also the case when you upgrade to a higher listing type or longer duration (e.g. Featured to Premium, Featured 15 days to 1 month). The original number of credits used to publish the upgrade will be discounted from the price.