Assemblies etc

I am trying to set up a simple stock control system but am having considerable difficulty.  A simple example of what we need to do is:  We sell things called marker rings either singally or in packs of five.  I set up a manufactured assembly - Marker Ring x 5  which includes includes five single marker rings.

If I now log a sales order for 1 set of Marker rings x 5 the stock of marker rings reduces by five but when I then deliver and then invoice the customer the stock increases by five back to its original level.

Processing an order for a single marker ring behaves as expected - reducting the stock by one all the way through.

If I use the direct invoice method the stock doesn't reduce either although again it does when single marker rings are processed.

Am I doing something wrong or is this behaviour correct?  If so is there a way I can achieve what I want.  I've looked at sales kits but it's no good displaying individual or calculated prices on the invoice.

Any advice welcome - thanks