There are two methods that look the same on your screen but have different consequences for what you want to do with the data.
Option 1 physically copies the data from your customer table into fields in your invoice table. If you then update a customer's info, those changes do not automatically appear in any existing invoices. The data is a "snap shot" of the data current at the time you created the invoice.
Option 2 simply links your invoice record to a customer record and you see the current customer information on your invoice. If you later change a customer's data, those changes will automatically appear in all past invoices for that customer.
Both options rely on establishing the same relationship linking the two tables. Define a field in Invoices called Cust ID so you can link the two tables like this:
Invoices::Cust ID = Customers::Cust ID
Option 1: Define fields in invoices for customer name and any other fields you want to copy from the Customer table. In Field Options for each field, select the looked up value option and select the matching field from Customers. Now each time you enter or select a customer ID number, the data from the matching info will be copied into fields of the current Invoice record.
Option 2: Simply place the desired fields from Customers on your invoice layout.
With either method, you typically would set up Invoices::Cust ID with a 2 column drop down list where column 1 is the customer number and column 2 is at least the customer's name. (Since two customers may have the same name, many developers create a calculation field in the customer table that combines customer name and other data such as their address and use that field for column 2.)