Same as always trying, I have never had Fitflops and never had any canvas shoes to stretch. Personally, I would not buy canvas shoes, hoping they would stretch. Too bad, because that seems like a good price for them. I know myself well enough that I wouldn't buy anything with a three day return policy unless I knew without doubt I would keep it. It would take me longer than three days to decide to purchase it, get it home, tried on, get other opinions, etc, etc and get it returned!
I do hope you find good fitting shoes that you like very soon.