Amazon offers Prime members the Suncast Premium 50-Gallon Storage Deck Box, model no. DB5000B, for $43.19 with free shipping. (Target charges the same. Non-members can spend $49 to qualify for free shipping; otherwise, shipping costs $18.79.) That's $12 under our June mention, the best now by $14, and the lowest total price we've seen. It's constructed from resin and doubles as seating. It measures 41" x 21" x 22".
Note that it's expected to ship in one to two months.