Copying the fields from the list box or combo is poor practice UNLESS you want to be able to modify them. For example. When you select a product for an order, you normally copy the unit price into the order. That allows you to discount it on the order if you need to. It also keeps the price on an order from changing if the price in the product table changes. If you are running a history report, you don't want something you sold last year to show with this year's price.
When I want to use a combo but show extra fields on the form, I modify the RecordSource query of the form to use a left join to the lookup table. That allows me to bind controls on the form to the lookup table. When you do this it is IMPERATIVE that you set the lookup control's locked properties to Yes to prevent accidental updates. So you have two situations, the price field will most likely be copied to the order details table but the product description will not. It will just be visible for reference on the details form.
Here's an example that shows the various ways to do this depending on whether or not you need to copy or just view the lookup data. Look at the 5th example.