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Amazon has 2-Pack NOCO Oil-Based Anti-Corrosion Battery Terminal Protector Washers (NCP2 MC303) on sale for $0.97. Shipping is free w/ Prime or on $25+ or $35+ orders (minimum requirement varies by location).
Note: This item is also available via Subscribe & Save for the same price (additional 5% off with +5 subscriptions). You may cancel Subscribe & Save anytime after your order ships.