However, for some reason, the value calculated by the formula does not update automatically. We already had the calculation options set to automatic and didn't know why values weren't updating.
Your answer is really a somewhat random reference to a different problem you had. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Then, when you open an account for a new customer you would have a Customer form to input a customer's data into the Customers table and an Order form to input the purchase information.
Later, you could print any number of Sales reports, grouping and arranging the information from the Invoices, Customers and Products tables to analyze daily or weekly or monthly sales in all kinds of combinations.
The fact that there is a different accepted answer means that this problem is different from yours and that solution worked.
Typically, the kind of error you describe (turning a value into the wrong type of argument for a formula), will produce an error message.