how to charge a customer for an item i just buy and use for installation w/o entering into inventory

When you write the check or enter the bill, you can use an item (set up as a non-inventory part) for the product purchased.  When you set up the item, check the box that says This Item is Used .... For a Specific Customer ...  and you can enter a sales price which is different from the cost.  You can enter it instead under the Expenses tab directly to an expense account, but you will not be able to track it by item this way.  Either way, under Customer:Job, choose the customer you wish to bill, and make sure the Billable box is checked.  When you invoice the customer, if the Billable Time/Costs window doesn't pop up, click Add Time/Costs at the bottom of the invoice form and select the Items or Expenses to be billed.