How about some new deal categories:
- Free shipping
- Price fall
- Bundled deal
- Stock clearance
- etc
These would generally truly be deal category rather than product category.
WHY
Because some of us may not be interested in some of those deal categories, e.g. as I live in a large metropolitan area, I generally don't care about free shipping (which typically means paying the same price I'd pay at a store down the road).
HOW
I see that tag implementation is problematic. So a second drop-down would suffice, on for Product Category and another for Deal Category.
WHAT
… to do with past posts? The category 'Sales & Promotions' can be reassigned as a Deal Category. Other posts can be left uncategorised, or automatically categorised using some simple filters, e.g. "free shipping", "stock clearance", etc.