On your "My Current Shops" page there is a link near the top "Submit an invoice". Click that and in the box put the job# of the audit or shop you did. You can add in any purchases you had to make for the shop, then click "Add". Just keep putting in the job numbers and hitting "Add" until you have all your work added to the invoice, then "Submit".
If you only do a few things for them now and again, it is probably frustrating.
But, I've done so much work for them now, I've gotten used to it.