No need to post the code here yet. From the looks of the error, there is a Mid function call on line 24 of page notify_shipped asp.
Go to that page and that line and make sure that you're calling the function properly. Remember, this is a function not a SUB routine, so it is supposed to RETURN a value, and that value is going to be of STRING type.
The syntax of the call is: S = Mid(string,start[,length]) where [,length] refers to an optional value.
Your call should look something like any of these:
strNewString = Mid(strMyString, 6) <-- returns all characters after character 6
strNewString = Mid(strMyString, 7, 10) <-- returns characters between 7 and 10
strNewString = Mid(strMyString, 1, len(strMyString-4)) <-- returns all characters in the string minus the last 4 characters of the original string. For dynamic string lengths.
Good Luck!