payment processor per product

You can achieve that via shipping methods.

Create two shipping methods:
one for real money products with a volume maximum of 0.99.
one for credit products with a volume minimum of 1.
Then, for credit products, add a width/height/length of 1.
Then, in your AUP payment plugin, restrict it to your second shipping method, and in your other payment plugin, restrict it to the first shipping method.

That should do what you want.

The AUP plugin is only in commercial editions.

Shipping methods can be created via the menu System->Shipping methods->manual shipping methods plugin.

Yes it will work. You can just hide the shipping methods selection by adding some CSS:

.hikashop_shipping_methods{ display: none; }