9. You can't return it, because it's one day past due.
已经过期一天了，因此你不能退货。
还能这样说：
You are not allowed to return it, because it has expired for one day.
It is not possible for you to return it, because it has exceeded one day expired date.
应用解析：
in due course 在适当时机；最终
10. I'm sorry we can't exchange it becasue this is our store policy.
对不起不能换，因为这是我们商店的政策。
还能这样说：
I feel very sorry for that we can't exchange it, because this is not allowed in our store.
I'm sorry our regulation doesn't allow to exchange the goods.
应用解析：
policy-holder 担保人，保险客户；
average policy 海损保险单；
policy on sth. （政府、政党、公司等的）方针，政策
11. Where do I go to return the goods?
我到哪儿退货呢？
还能这样说：
Is there any place I can go to return the goods?
Can you show me the place where I can return the goods?
12. We regret that these goods are excluded from the rejection.
很遗憾，此商品不在退货之列。
还能这样说：
We are sorry that these goods are not in the list of return commodities.
We regret that these goods are not guaranteed to return.
应用解析：
exclude...from 把……排斥在外；不准进，拒绝
13. The premise of refund for this is that the goods must be in good condition.
退货的前提是物品得完好无损。
还能这样说：
The precondition is that the returned merchandise must be safe and sound.
The premise of returning the goods is to make sure it is in perfect condition.