Since you have no receipt, do you have any other images that were required for the shop, i.e. business card, brochure, etc.?
To invoice...
On the first screen of the invoicing process, enter 0.00 as the amount of reimbursement, and write "No Purchase" in the Notes Field.
Next, and I'm sure that I'll be corrected if I'm wrong, but you could submit your Proof of Visit image as your receipt image.
If there was no image at all required, simply select the option that is "I will mail in my receipts." But don't mail anything in. They will take care of it.