Id doesn't matter if you will use ajax or not. Always user will have a way to change the data that he is sending to you. The ground rule when you develop applications is never trust the data from the user.
In this case the trick is to just gather main data, so item id and amount. You don't take prices from the form. Before sending you check the prices of chosen products in your database and send those to Paypal.
So the only variables that he can modify are product and amount. If he will change the amount from 1 to 10 he will just pay for 10 so it's not a problem :)
I don't know this class that you are using but it's very unsafe, and you shouldn't use it. You send data to Paypal using curl for example and not directly from the form.
You should read how to use express checkout or direct payments.