unique user id tie in with paypal

hi guys

i have a website with a service on it which works without a login system. i want to charge for it through payment gateway, how do i ensure that each session for the user is unique when multiple users access the same service?

example if user A inputs 2+2=?, then website redirects him to payment gateway after which result 4 is shown.
but while user A completes his session another user B comes in and wants to know 3+5= ?, say if i have a 100 users simultaneously starting sessions wanting to know their results how do i go about handling them?