it is a bit tough to recall how it was done in 8. The order structure doesnt not exist till you redirect to Paypal. On your last step you in your paypal payment provider before redirecting you convert the shopping cart to order. After that the shopping cart structure does not exists.
I suggest you watch developing custom payment gateway. it will shed light on all this. Although it says version 6 - most of it will be valid for version 8. See kentico 8 documentation on custom payment gateway as well. These two links should give the understanding how all the process works.