The way I've always done it is to set up some standard items that are used as non-inventory items. You can set them up by account if that's what they know. For instance, you can set up an item called "VALVE" and have a default account assigned to that. The inventory type will be Services or Misc Charges or Flat Fee. You can still edit the description each time if needed. It helps with account coding and consistency of what the invoices say.
You can also set up a default account system-wise under Tools - Setup - Posting - Posting Accounts.
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