I have a custom object that we use to track our business from order through payment.  Part of that tracking includes what items are shipping, in what quantity and at what price.  I'd like to use the same functionality as our sales team uses on the opportunities - ADD PRODUCTS.  That works exactly as I need this to work, with a list of all products, the ability to select the relevant ones, enter a quantity, an extended price and track any comments as well.  I just can't seem to figure out how to get that functionality on a custom object.  

I considered using a different opportunity record type, but that won't work for a variety of reasons.  

So, is there a way to put the "Add Products" functions on a custom object?