Poor service from Zando

My friend and I purchased 2 pairs of Superga (mushroom coloured) sneakers from Zando (Order number ********** 98), using her first time purchase 30% discount (APP30). When we received it, we noticed it is too big for us both, which is strange because we wear size 7. Anyway, I contacted Zando, wanting to exchange it for size 6.5. I was told there is only one pair of the same sneakers available in mushroom. I said that is fine, they can then send us one mushroom pair and one black pair (for which we will pay in the difference), just not wanting to lose the 30% discount. I was told by Nurjahan that is not possible as it has to be exactly the same item. I understand it, but firstly it is not our fault that the size 7 doesn't fit (I have ordered size 7 shoes from Zando before without any sizing problems - my order numbers: ********** 48, ********** 29, ********** 56, ********** 76.) and secondly it is not our fault that Zando doesn't have 2 pairs of size 6.5 sneakers in the mushroom colour available.!!! I then suggested that we do a return, getting the store credit for the amount we paid and then order the size 6.5 in mushroom and in black, but still with my friend's 30 % discount, because as this is then a return, she hasn't used her 30%. Nurjahan then told me that will not be possible, we won't be getting the 30% discount then. So what do you suggest??? I have always recommended Zando to all my friends, and because of my recommendation, my friend decided to try buying from Zando. And this is the service we get??!!! I will definitely not be recommending Zando to anyone ever again. And I will never buy from Zando again!