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Amazon [amazon.com] has 18.5L Travelon Anti-Theft Active Packable Backpack for $24.06.
Shipping is free with Prime or on $35+ orders.

Amazon [amazon.com] has for Prime Members: 18.5L Travelon Anti-Theft Active Packable Backpack (Charcoal) on sale for $21.20. Shipping is free.