New to CRM and have some questions regarding invoicing

I am very new to Microsoft CRM and have purchased the hosted solution from Microsoft to aid my small consulting firm.   We have a wide range of more "services" than individual "products".   Our primary "service" is software development and database administration.   So understanding how to invoice specific people in CRM has sort of gotten me a little baffled.   It appears that the only way to actually assign a price to an invoice is via either an Existing Product or Write-In Products and/or I am just not seeing how to do this.   The whole idea of getting CRM is because a rep sold me on the idea that i could schedule in my developers time, and then be able to generate an invoice based on the time utilized.   Now i'm feeling a little discouraged about the product and almost feel a little "taken" because its not as easy as it was sold to me on so my questions are:

1)  When a customer requests development should my developers log this as a "Case" or an "Activity"

2)  In regards to question 1, when I go to generate an invoice to my customer can I assign a value or price to the case or activity?  We work solely on billable hours to a client, not selling a particular product like SaaS or anything like that.

3)  Primarily the dropdown items for CRM offer what we need, but in some regards I would like to make some slight modifications such as we invoice NET15 and CRM is only offering me NET30 or higher.    

I really need some help here, and I know that EE is not the place to search for hireable consultants, but this has become pretty cumbersome to me.   i really do like the product, and a lot of things about it, but I need some help on the setup of the product.