It's difficult, because the developers surely want to create an order system that works with lots of different scenarios.
However, for us, that vast majority of our customers have very very basic needs and so something that is extremely simple is necessary (critical).
1. Customer searches for domain name.
2. If new domain name, then choose the big "BUY NOW" button. If domain exists, then ask if they own it and want to transfer it. Otherwise, redisplay the search box, and also maybe show suggested names. Repeat as necessary until eventually they have selected a domain name, and we continue.
3. Show all the hosting plans and ask the customer to pick the one they want. Customer chooses the plan, and we continue.
4. Ask the customer what billing cycle they want for the hosting plan. (Monthly, bi-annual, yearly, etc)
5. Done. Press "Pay Now", and complete payment process.
A. All of this should be on ONE page that uses basic AJAX to refresh, and has tally in the top-right area. Each "section" of the order process (domain selection, hosting selection, billing term selection) is displayed as a new area below the previous section.
B. If you want to allow addons to be ordered during initial order process, then they can be displayed as part of step 3, depending on the plan they select. Again, keep it all on the same page. Simple is better. (We actually don't even allow customers to pick addons during their intial order, because it was way too confusing for them to see so much stuff they had no idea if they needed or not. It often prevented them from completing the order, which is against basic good business, right? So now, we simply let them pick their domain and hosting plan and that is it. AFTER they complete their order, they can then add addons from within their customer account client area. By then, they already paid, and are "invested" into things, as opposed to being overwhelmed and not completing the order.
I hope that helps. If you are looking for some good examples, check out Hostbill's order pages. They have some amazing order screens and processes that we found to be very nice.