Ok, so this is not a reimbursement from an employer as I had originally thought. 🙂
What you need to do is report all your income and expenses as Self-Employed on a T2125 - Statement of Business Activities. Your income is going to include the "invoices" you sent the American company, and your expenses will be the actual receipts which you have kept.
If you had submitted the receipts to the company and received a straight across reimbursement, you would need to do nothing as they would be using the actual receipts as their own expense or write off, and you would not have been considered to have received any income.
