TOPIC: Multiple user defined donations

I have a customer that needs a donation option that they can have 15 different donation options but the donator needs to be able to define their own amount and just be able to pay one payment. Is this an options yet that it will auto calculate or does the user still need to calculate it on their own?
Even if the donator has to do the calculation this may fit what I need to take care of their problem.

If I could get an answer to this as soon as possible I would appreciate it. I need to get this project done for my client.