Of course, for a composite key to be useful it has to be unique, ie only one possible record with that combination.
As soon as you start adding other fields like amounts or dates which could give you the possibility of having more than one record with the matching combination of fields the key ceases to be unique and so can't be used as a Unique / Primary key, but could still be useful as an Index for sorting.
It would probably not be sensible to use amounts, or dates / times as part of a key unless you could guarantee that:
a) they would never be duplicated, as part of the combination of fields
b) they would never be changed.
So if you have the possibility of having multiple records with the same groups of fields, excluding amounts or dates, then you will probably have to use something else, ie an autonumber or generated field, as your key.
In the OPs case the table with the fields EmployeeID & CustomerID could not be considered as unique because there may be more than one order from a particular customer placed with a particular employee.
If there were a repeat order it may even be for the same amount, so that couldn't be used.
If the customer was a big company theoretically they could make more than one order, with the same employee, on the same date so that wouldn't be unique either.