Read this article on Payment Types to set up alternate forms of payment.

When you receive an invoice from your Vendor, the AP Voucher is the tool we use to record the payable and get it into the queue for payment. Vouchers are used to reconcile invoices from Purchase Order receipts as well as direct expenses.

If you've created a Purchase Order for Inventory items then you'll need to get those items into Inventory so you can sell them to your Clients. Here's how to do it!