Questions related to the use of a third party product is going to have to go through their support. This forum just focuses on MS products so we aren't going to be much help.
The most I can say, having used PayPal for many years, is that everything you are asking to do can be done by PP easily. All you need to do is hook your buttons up to a controller action. In the controller action create a transaction on the PP side for the correct amount, this is what PP uses anyway. You'll get a token back that you use to redirect to PP such that it can pull the correct information. This is really no different than what you're probably doing now.
However, as I already mentioned, PP will need to provide support for the specific details of getting this to work. But it shouldn't be any different than what you're doing now. The amount is a configurable option set when you create the transaction. Where that value comes from is irrelevant.