Have you considered using reverse SMS billing or direct mobile billing. One of these two would provide you with a faster way of charging for your service, without the hassle for the user. Reverse SMS is pretty easy and quick to implement, though it might lead to bill shock so be careful.
Parkers car valuations is a good example of this type of service. You input your registration information and they provide an immediate valuation for 1.50 charged via your mobile account.
You may also find wiki's article on mobile payments interesting as it outlines the most popular mobile solutions in the field.