hoping this subject will draw a reply. I posted a similar request on 3/3 but no replies.
I'm using standard costing.
 
according to the help file:
AP-C or PO-N lets you compare purchase order prices with your vendor invoice prices.  If purchase order prices are incorrect, they can be changed from within this program, and the inventory last cost and average cost will be updated if the items were purchased for stock and any job costs will be changed accordingly if the original PO receipt was to a work order.  
 
My average costs don't update; only last cost.  I have been able to get it to work in my test company twice yesterday but then failed several times on the same part number.  I can't seem to get a handle on the circumstances when it works.  I have an example in the 3/3 post.  
 
Bottom line is that IN-L-E can't use Average cost.  
 
Can someone who uses Std Costing tell me if you see the same behavior.  And is this the correct behavior?  
 

Colin is correct.  If your costing method is set to Standard, then any corrections to cost in AP-C as a result of the vendor price on the invoice being different from the PO will post to the Purchase  Price Variance, not change the item Average Cost.