There are differences between the terms "service" and "web service".
A web service is an integration technology for connecting applications. It is not an implementation technology for executing functionality or business/domain logic.
A service, in the context of SOA, is a loosley-coupled, independent function. And there are many different design styles for services. They can be fine-grained, coarse-grained, data-oriented, logic-oriented, etc.
Unfortunately, the fact that these two concepts use the word "service" may be the cause of confusion and misunderstanding.
This depends a little on the scope of your assignment. If you only are going to develop a single web service, then pretty much anything will do.
However, it would be much more interesting to develop services in, for instance, two layers; the lowest service layer contains more fine grained service, while the layer above contains orchestration services which uses two or more services from the lower layer.
It would be interesting to hear what your plans are regarding the accounting example.