For in-store pickup only, Lowe's offers two 20-lb. Bags of Kingsford Charcoal Briquettes for $12.99. At $6.50/bag, that's the lowest price we could find for this quantity by $10, although we saw it for $3 less last summer.
Update: As a reader discovered, if you order four bags, you'll receive an . extra 20% off in-cart. That cuts the price for all four to $20.78 ($5.20 per bag). Plus, you probably won't need to buy briquettes for a long, long time