Firstly this is not for a webshop, but it is for a page for our product.
The requirement for the joomla extension is that calculates a price based on how many users, customers our client wants and which add-ons the client wants.
So our client should see a calculation like this:
3 users $x pr. month
n additional users n times $y pr. month
m each additional 100 customers m times $z pr. month
This gives a total pr. month
Then there are setup (once only) fees
Credit card payment setup $y
And then at the bottom a total for the first year(!) which includes the once only fees and a total for following years (without the once only fees).