That's a toughie. For instance, I actually have four sets of BP batteries, 3 total chargers, a couple sets of VVPV batteries,... Automatically receiving another charger and batteries any time I buy another BP makes no sense for someone like me; it's great for first-time buyers, a shrug for repeat customers.
A discount on charger/battery combo at time of purchase makes sense, but I believe (could be wrong) that notcigs already does that regularly.