The credits, depending on your accounting methods, could be used as a tax writeoff since it's technically marketing, and technically a gift, and technically lost revenue. There's a number of ways of writing it off, but as always, your accountant will know the correct method for your state/city/location.
Also, your accountant will likely have a prefered accounting software that they'll suggest to you. While Quicken is great from home users, most accountants hate it and would recommend QuickBooks or other software. But again, if you're going with a simple accounting method, you could ignore all software and all tracking, and strictly pay your taxes based on the income deposited to your bank account.
When you prepay to domain places (enom etc), that could be considered as an asset, a float account, or separated into a separate credit account entirely. It'll fall under vendor too, so it could be considered as a payment to a vendor.
Again, depending on your accountant's methods, using it as a vendor account could be written off as a COGS. It can be considered as either a Debt, or a Credit, but it depends on how you're going to be tracking things.
Have you spoken to an accountant yet?