Stopping bots on checkout

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I have a shoe store that sells limited products. When I post them online they are bought up by bots with auto-checkout then most orders are canceled if they don't want the shoe and real customers never get a chance to buy because more bots buy them when they are relisted. How can I add a captcha like system to prove people orders are not from a bot without paying for Shopify plus which is too expensive for me at the moment.