I’d need to add a Paypal ordering form where it should be possible for customers to choose a service (I am not seling goods) offered in many different options.
Say, for instance, to have displayed:
Service A, and then a field showing a drop down menu with Participants Number 1/2/3…etc for it, with total selling price displayed accordingly with the amount of participants chosen.
Even better if I could display multiple choices both in Service field and Participants field, and then have a total price automatically calculated accordingly with different combination of choices.
Does it exist a plugin offering such features?
It sounds like what you need is an ecommerce plugin. Take a look at WP e-Commerce. It can do PayPal as the payment gateway, and I believe can handle multiple product variations (Participant Number and Service would be variations).
There are a bunch of e-commerce plugins. In WordPress, you can go Plugins -> Add New and use the search box to search for “e-commerce” and come up with a bunch including ratings.