First of all, you as an affiliate get net commissions.
Example: if a product is $100 and the commission is 50%, you get $50 net. You do not pay any PayPal fees at all, the seller is the one paying the PayPal fees for you.
However, when commissions start to approach the 100% mark, the seller can't pay PayPal fees for you because if he does so, he will go negative. Taking the same $100 product example, he can't pay you $100 because then there will no more funds left to cover for PayPal fees and Zaxaa fees (if any).
Thus, for products with 100% commission, you as an affiliate get 100% of whatever is left AFTER PayPal and Zaxaa fees (if any) are paid.