Re: "In the past I would enter sales commissions in the check...
"I can't enter an invoice or a sales receipt for an entity on my vendor list. Invoices and Sales receipts only apply to Customers, correct?"
Anyone who pays you is your Customer. A Vendor can be both a vendor name and a customer name for the financial activities between the two of you, and you must Honor and use the right name for each function, as if that is two different entities.
You pay and report vendors for goods and services, for 1099-Misc. Not the Customer Name.
Customers pay you for sales of goods and services, and the commission income is paid to you by a customer name. Not the Vendor Name. Yes, you would use invoice or sales receipts.