I want the item code given by my vendor to print on the invoice automatically. How can I configure BUSY to print the vendor item code on invoices?

If you want to print the item code in the printout of BUSY that is sent by your vendor, you will need to follow a few steps. 
Step 1: First, you need to define the item code within the item alias. To do this, go to Administration, then Master, and click on Item. Next, open the item master in which you want to maintain codes and mention them in the item alias option.
Step 2: If you are using the standard format of invoices for printing, go to Administration, then Configuration, then Invoice Document Printing (standard). Click on the sale invoice option, then on the right side, there will be an option of "Self Configured." Click on the "dot option" beside its name, and define the character in front of the item alias, ensuring that the total width should be 136.
Step 3: If you are using the Design format, go to Administration, then Configuration, then Invoice Document Printing (Advance). Click on the sale invoice option, then from the Header and Body tabs, you have to configure it.
By following this process, item codes that are defined in the item alias will start to reflect in the sale invoice printout.