14. You can change the commodity within three months.
3个月内包换。
还能这样说：
The commodity can be changed in three months.
In the period of three months, you can change the commodity.
15. I ask to return the goods, for there appears some quality problem.
因为质量问题，我要求退货。
还能这样说：
Because of bad quality I need to return the goods.
Since there exists some problems in quality, I need to return the goods.
16. I want to change a larger one.
我想换个大一点的。
还能这样说：
I need to replace the goods with a larger one.
I want to transfer it to a larger one.
17. If you return this, we will deduct 5% of the price.
如果退货，则要扣除5%的钱。
还能这样说：
In case that you want to return this, 5% of the price will be taken off.
The cost of the returning is 5% of the price, if you want to get a refund for this.
18. I want to know the guarantee period to change the goods.
我想知道换货期是多长时间。
还能这样说：
I want to know how long you can warrant me to change the goods.
I want to know the warranty date to change the goods.