Your entries should flow to Sch D from F.8949 with Code C reported in column (f).  This will cause the gain to be subject to a max rate of 28% instead of the usual LTCG rates.  You won't have to attach the invoices but should ask your client to retain those in case they get audited.  Assuming these transactions were carried out in CAD, spot rate would be used for conversion to USD as the functional currency.

In addition to cap gain, you may also need to account for §988 gain, which provides for a $200 exception; again, my presumption is that your client was paid CAD and would exchange that into USD at some point.