英语翻译One of the rules is that we are not responsible if the s

英语翻译
One of the rules is that we are not responsible if the school post office happens to misplace your check should you decide to put it though the mail,which it appears you did.
One of the rules is that...后面是表语从句
we are not responsible if the school post office happens to misplace your check 这里是条件状语从句
请问should在这里是什么语法?