I would also consider and register onetwoshop.com and/or getonetwo.com (good for specific product/service) for protective purposes. Alias the domains to redirect to whichever you choose anyways, and consider running some A/B testing with adwords or social media posts to see which your target market respond to better.
I would use the success screen, primarily because it can help the conversion rate.
PRO success screen:
focus of purchase screen is to review and pay
keeps the purchase summary clean
helps conversion rate
in my experience customers pay attention to the success screen
similar pattern: I've seen cases in which customers can still create an account on the ...
Is there ever a situation where this stepper is active? If not, then this is not the appropriate control. Why is the inactive stepper shown at all?
If a lot of other things can be bought and only the artwork has this restriction, then the inactive stepper could work (though I am pretty sure users will be confused why the cannot change the number for just ...
You could go with assumption it is quicker for users to check information right away, as it is simpler to compare products when they are next to each other.
If you have strong established practice and know your users are satisfied with existing state, you don't need to do much.
What you could to i.e.:
In a questionare / interviews / with analytics, find out ...
Yes, because users aren't going to be constantly checking back on your website just to see when their parcel is going to be delivered.
Some kind of "push" notification - email or text message - will alert the customer.