You could always go to the bathroom and into a stall and write them down or take pictures.
Take the customer's card into the bathroom?
In keeping in the vein of simplicity and pragmatism, you could just keep a calculator under the counter. After turning on the fax machine and killing the transaction, scribble down the expiration date. Announce, The fax machine must be on, the interac machine doesn't work when they're both on. How am I going to turn this thing off.
Now with a pretense for pressing buttons, key in the first numbers into the calculator. Once the screen is filled, add it to memory and enter the remaining numbers. Turn the fax off, finish the transaction. Put them numbers down with the expiration date and put the paper in your pocket.
You could do this in a few steps, to minimize any suspicion about hitting a peculiarly high number of buttons, but the longer you spend the more chance of your boss checking up on you.