Upload & Sell: Off
Like... I just emailed her this.
In lieu of another such "cost based solution" or "deduct this from that" solution, and even though you wrote her back already, here's a thought:
A third alternative. What if you were to offer a more customized solution?
Because when you do that, you can charge more than just for a file. Because it's no longer just a file. It's a file custom made for her situation. It has added value to the one it's customized for.
If you were to say to her something like, "hey cool you wanna make a canvas yourself, ok... I've got a lot of experience creating images on canvas so they come out looking awesome, so how would this be, if I finish the file, you know get the resolution for best printing on canvas, and do it specifically optimized for the size you want, sharpen it for output on canvas, get the composition aligned so it sits just right on the frame and looks good, all that... and it's $150." I'm using $150 purely to illustrate.
Now, maybe the price of your canvas less its cost is the same, also $150. Assume it is. Which way of getting to that $150 do you think a customer might justify spending more on for a file?