For in-store pickup only and with stock varying by ZIP code, Walmart offers the LEGO Classic Creative 303-Block Supplement Set, model no. 10693, for $13.99. (Target charges the same with in-store pickup; Amazon charges the same with free shipping for Prime members.) That's $2 under our mention from last December and the lowest price we could find by $4. It features 20 colors of extra blocks in many shapes and sizes.
Update: Specs corrected.