[Translation from Japanese to English ] Since the price of bigger sized shoes and smaller sized shoes are different, ...

Original text

靴の大きいサイズは、小さいサイズと販売価格が違うのでどちらとも同じ価格にして頂ければ、ご提案頂いたディスカウントの価格で購入します。なぜなら、大きいサイズは、購入希望の総数量の割合からは少ないので、小さいサイズの同じ価格の方が、販売しやすいからです。そして、購入する商品のサイズが、欠品しないようにして下さい。
また、サンダルは300円なら購入します。なぜなら、同じ商品が、インターネットですごく安く売っているからです。

Translation / English

Since the price of bigger sized shoes and smaller sized shoes are different, if you make both of them the same price I will purchase with the suggested discount price. To give you a reason why, it's because bigger size have less needs so it would be easier for me to sell them for the same price as the smaller size. Also, please make sure you have all the sizes I'm purchasing stocked.
Furthermore, I will purchase the sandals if they were 300 yen because I found the same item being sold much cheaper on the internet.

Translation / English

The price for bigger size shoes are different from the smaller size so, if you could make them the same price, I will purchase at the discounted price you proposed. The demand for the bigger size is smaller and it's easier for me to sell the smaller size. Also, please make sure to maintain inventories for the sizes I purchase.
I will purchase the sandals if they are JPY300 because the same item is sold at a very cheap price online.

Translation / English

The price of larger size of shoes is different from the price of the smaller ones. I will buy the larger ones if you make it to the discounted price which you offered with the smaller ones. Because the percentage of sale for the larger size is not as high. It is easy to sell smaller ones in the same price. And I want to have the shoes that I will purchase enough in stock.
As for sandals, I will buy them with the price of ¥300. Because the same items are in sale with really cheap price in the internet.

Translation / English

If you sell large size shoes and small size ones at same price, I will buy them at the discount price you showed. Becasue we have a small percentage of sales of the large size shoes and can sell small size shoes more at even the same price as large size ones. And please keep the shoes I want and do not sell off.
Also, I will buy sandals at 300 yen because those are sold on the internet at very low price.

上記のように英訳させていただきました。
お役に立てると嬉しいです。