I'm pretty sure that Carl (Verve) only charges a £5 deposit, and that's just to cover him in case someone tries to waste his time by asking him to do stuff and then ending up not paying him.
Vistaprint uses a different business model to most graphic design companies, in that they are very much a "DIY" service using a number of pre-set templates (or the ability to upload your own images etc)... so it's just you and their server until you decide you're happy and want to go ahead and purchase them... so it's not a question of another person having to do any work... VistaPrint's server does all the work at your bidding, for probably a trivial amount of electricity and network bandwidth too - which is a lot cheaper than paying the salary of a skilled graphic designer!
But it's a question of "you get what you pay for"... it's just you and VistaPrint's server if you go with VistaPrint, so you're not paying for a second pair of eyes to review and check what you've done, nor are you paying for the services of a qualified graphic designer. And (unless you upload your own artwork) you'll probably be using the same template as countless other businesses.
Whereas with a graphic design company like Verve or Bimbo, you are paying for a qualified graphic designer to do the work for you... who will be able to advise on what works and what doesn't, and come up with a bespoke design for you rather than re-using a generic template. Hence why the cost will generally be more than VistaPrint, as you're paying for someone else's time and expertise... and hence why you'll normally be asked to pay a deposit (as many small businesses don't want to risk offering people credit or risk them not paying up in the current economic climate)...