判断一下我的翻译是否正确,如果有更好的翻译请指正!谢谢!

But the grocer must charge you more than the big market for most items because he can only buy a small number of cases of one product from the manufacturer, while a supermarket chain can buy thousands of cases at a time; thus the supermarket can buy cheaper and can sell cheaper to you.
可是杂货店的商品要价必定比大超市的高,因为他只能少额地从厂家进货,而超市连锁店(别扭)却一次进货成千上万种(case不知道怎么翻译更好),因此超市进价便宜卖给你也更加便宜.

可是杂货店的商品价格肯定比大超市的高,因为他们一种产品只能从厂家进少量几箱,而连锁超市却一次进货成千箱，所以进价便宜，卖给你也就比杂货店便宜.
这里cheaper不是更加便宜而是比杂货店便宜。