Cost Estimate Type user event script question?

I've made a basic script, user event before submit that runs on sales orders.  We have a custom markup field we use for certain customers, so it just applies a cost + x% markup to all items.  The script first looks for an average cost and, if it has one it uses that value.  If not, it uses purchase price from the item record.  What's happening is, the script changes the cost estimate type to average cost.  In the ui, this would automatically load the new est. unit cost from the item record for the average cost (for that particular