Restricting Updates Of Product Prices

I am trying to set up a database to track truck maintenance, repair, and inspection records for a trucking company. Parts are ordered from vendors, and I have forms set up to facilitate the entry of the information regarding those orders. The problematic form is based on a query. In that query (this form and query deals with the purchase details - product, price, extended price), the product ID is gathered from the inventory transaction table, the product name and price are gathered from the product table (I did this so that price would enter automatically).

Of course, when I change the price, it is changed across all related orders, including those that have been saved previously. I know that the problem more than likely stems from having the info pulled from the products table. I'm just at a loss as to any other option to have the price automatically pulled.