Shall：必须。用来表达强制的，硬性的要求 (mandatory requirements)。不达标、不执行则被视为违规。
Should: 应该。用来表达期望、建议，或指导性的要求（guidelines，recommendations or desired items）。做到了，客户很感谢，做不到、不执行不被认为是违规。
一般来说，在商务合同里面表示必须基本都用到shall 而不是should，合同本身就是一个规定了权力和义务的法律文件，如果用should 表示必须，那在法律上就有瑕疵了，失去了约束力。
The word ‘Shall’ has the following meanings: An imperative command; has a duty to or is required to. For example, the notice shall be sent within 30 days. Usually ‘shall’ used here is in the mandatory sense.
“Shall”一词有以下含义: 命令;有义务或被要求做。例如，通知应在30天内发出。通常这里的shall是强制意义上的。
The majority of agreements, contracts and legal forms continue to use the word shall. Instead, these documents should be drafted or revised to use must, may, will or should.
大多数协议、合同和法律形式继续使用“shall”一词。相反，这些文件需要先起草稿或修改，则可以使用must, may, will或者should。
The word should does not express a legal requirement. But shall, in the case of the climate agreement, does. Shall we continue? We often use the auxiliary verb shall to indicate a promise .
“should”一词并不表示法律要求。但就气候协议而言，就必须使用shall一词。我们继续好吗?我们经常用助动词shall来表示承诺。
商务合同条款翻译例子：
例子1：合同总价应以美元支付，通过ABCD银行（下称“买方银行”）和卖方指定的商业银行（下称“卖方银行”）进行。
译文：The total contract value shall be paid in USD, through ABCD BANK (Hereinafter referred to as “Buyer’s Bank”) and the commercial bank appointed by the Seller (Hereinafter referred to as “Seller’s bank”).
注释：法律英语（商务合同）中“下称” / “以下简称”翻译成：hereinafter referred to as。举个例子：
出租方(甲方) Lessor (hereinafter referred to as Party A) ：
承租方(乙方) Lessee (hereinafter referred to as Party B) ：
根据国家有关法律、法规和有关规定，甲、乙双方在平等自愿的基础上，经友好协商一致，订立本合同。
In accordance with relevant Chinese laws, rules and regulations ,Party A and Party B have reached the agreement through friendly consultation based on the principle of equality and voluntariness.
例子2：合同总价的40%应在买方收到以下单据并审核无误后15天内通过电汇付款给卖方。
译文：40% of the total contract price shall be paid by Telegraphic Transfer within 15 days after the buyer has received the following documents and check them。
例子3: 买方应在定金付出后20天通过买方银行开立合同金额60%的以卖方为受益人在不可撤销信用证，凭以下单据付款：
译文：The buyer shall，throug the buyer’s bank within 20 days after the down-payment open an irrevocable L/C covering 60% value of contract in favor of the sellers and payable against the following documents。
注释：down payment：定金。
拓展学习：down payment ：首付款; 定金; （分期付款的)首期付款; 预付金
英文释义为：If you make a down payment on something, you pay only a percentage of the total cost when you buy it. You then finish paying for it later, usually by paying a certain amount every month.
举个例子：
If you’re borrowing money for the down payment, provide a copy of the entire loan agreement. 如果你要借钱付首付的话，要提供一份完整的贷款协议副本。
凭单据付款：payable against documents，信用证用语中都是against 这个介词表示：凭借。举个例子：
We usually accept payment by irrevocable letter of credit payable against shipping documents.
我们通常只接受不可撤销的信用证，凭装运单据付款。
via 英语汇编
