Hi MartinT ,
Thank you very much for your post.
Our Product Levels premium add-on should just work fine for you.
Let me explain:
Your sandbags are basically a product part as I am not sure if you pack them any further etc.
1. So all you need to do is install the above mentioned add-on and then create a product part type of product. You enter the quantity of all your stock in there.
2. Then you go into your already created variable product (I guess it is variable, right?) and create a connection using the Bill Of Material interface to your product part. So for example variation of 10 bags will have a connection to 10 units of your product part and so on.
When one of the variations sells, the correct number of parts will be removed from the product part stock.
Important is to try the demo and create your scenario. And of course, please, do read the full documentation.
Hope this helps.