如何从语法角度理解这个英语句子They will tell you that's how much you have to pay.其中that和how都是宾语从句的引导词吗?是否可以省略‘that's'?

不可以省略。
如果省略了that's 整句话的意思就变了
不省略that's 的意思是：他们会告诉你这就是你要付的价钱。这句话已经省略了宾语从句的引导词that,原话应该是：They will tell you （that） that's how much you have to pay.
省略that's 的意思是：他们会告诉你要付多少钱
that how 都不是宾语从句的引导词，that是主语。
如果还不明白，可以发消息问我^_^

无所谓!
保留that's时
省略了从句引导词(that), 从句本身是主系表结构(表语本身又是个从句), 即从句中包含从句
省略that's时
从句即是how much you have to pay

原句的意思是，他们会告诉你那（that）就是你要付的钱数。这里面that's=that is, that是主语而非引导词 所以不能省略
去掉that's 句意就变了 they will tell you how mcuh you have to pay. 意思是，他们会告诉你要付多少钱。把that's 变成that也是这个意思 they will tell you that how much you have to pay. 这个that是引导词 可以省略

可以省略that's ,再说用that's 也是不对的，只能用that，而不是that's

that's how much you have to pay是整个句子中tell后面的内容,即告诉某人什么事,是tell后的宾语补足语
补足语中,that是主语,is 是系动词,how much you have to pay是how引导的表语从句
他们会告诉你,这个数就是你要付的钱数.