Especially since you have the screenshot, I think you need to email them and make sure you get that 5% back since you did not authorize the discount.
My friend bought a bag awhile ago when their system was having some weird glitch. When she checked out, her Chanel bag showed $1850 was to be charged but in their confirmation page, they charged her 10% more. She was curious about why the checkout page showed $1850 but on their listing it was 10% more so, luckily, she screen shot it and sent it to me for advice. I told her if the price is cheaper in the shopping cart, it's a win-win for you. So she went ahead and completed the order only to have Tradesy charge her the amount on the listing. She emailed them about it with the screen shot, stating that she only agreed to pay what was on the checkout screen and they had to honor that price (after telling her there was a glitch in their system).
I don't see how they can talk their way out of this one especially if you have that kind of proof...