1）凡購買XX貨品，即可以優惠價＄10換購朱古力一盒。

Whenever purchase of xxx product then get one box of chocolate at a reduced price of $10.

2）凡購買XX貨品，可以加＄10購買朱古力一盒。

Whenever purchase of xxx product then may get one box of chocolate by added $10.

1) Purchase XX goods, which can be redemption price of $ 10 a box of chocolates.

2) Where to buy XX goods, you can add $ 10 to buy a box of chocolates.

1. Buy XXX, can redeem a box of chocolate at $10.

2. Buy xxx, add $10 to get a box of chocolate.

As a supplement to ilovechopinpiano's answer:

(1) Buy xx and get a box of chocolate at a discount price of $10.

(2) Buy xx and get a box of chocolate with $10.

Hi!:)

You might consider:

1) Buy XXX and get a box of chocolate with a discount of $10!

2) Buy XXX and get a box of chocolate with an addition of $10!

翻譯為英文

1) Purchase XX goods, which can be redemption price of $ 10 a box of chocolates.

2) Where to buy XX goods, you can add $ 10 to buy a box of chocolates.