Why do my credit notes always have positive amounts?
A credit note is used to partially or completely offset (read: refund) an invoice.
An invoice is always a negative amount, e.g. charging someone $50 for a fine pair of hand-crafted shoes that means they must pay you $50 (-$50 from their account).
A credit note is therefore always positive, because you are returning money to your customer. In the case of the lovely shoes, this would mean that their account is now +$50, and you are plus one mighty fine pair of artisanal loafers that perhaps you wanted to keep anyway...