Work orders are, for all intents and purposes, a sale. Instead of selling a product, you're selling your service to repair a product. Whenever a work order is made, unless adjustments are made in the cost of products and labor of an item, there will be payment required for the repair work.
You can tell at a quick glance which work orders require payment. When you navigate to the Work Orders page, you’ll see in the queue the work orders marked as Collect Payment.
When you click the Collect Payment button, you will be brought to the regular sales page you see at checkout. You can adjust the parts and labor on this screen as well as enter payment information like a normal sale. For more information on the sales screen at checkout see the documentation here.
Once a work order has been paid in full, the option to collect payment will disappear in the work orders queue. Note, however, that the status of the work order will have to be manually changed. See the documentation here for more information on work order statuses.