Hi Suzy,
You want to check the security role(s) for your sales rep. If you're using the standard "salesperson" security role, he/she won't be able to use the "write-in" feature nor can they change the price of an item from the product catalog.
To enable this feature, check the security role, under Sales - Miscellaneaous Priviliges and change "Overrride Opportunity Pricing" to the correct setting. Changing this will allow the use of write-in products and changing the product prices. As you will see, you can seperate this from Order, Invoice and Quote pricing security.
Keep in mind that when you change the security role, other CRM users might use the same role. If you don't want this to happen, make a specific role and add it to the correct user.
Hope this helps!
Regards,
Steven