Assigning order to user

Everything working fine so far, but one thing keeps me
struggling.

How can I assign a specific user to an order, so that I can
update this
order in my system after I get the "CHARGED"
message from GC.

The flow I'm thinking of is:
- user creates cart by adding items.
- goes to checkout-page.
- cart is submitted to GC and at the same time stored in my
database.
- when I get the notifications I can update the specific
order.

but how do I identify the orders ?
for example if multiple users drop carts to GC ?

can anyone give me a hint.. ?

thx...

ok - if you have a similar problem:
you can solve it with merchant-private-data >
merchante-note

greetz