The problem
Your carrier won't deliver to PO boxes, but Shopify's checkout accepts them anyway. Every one that slips through becomes a refund, a support thread, and a shipping label you paid for twice.
The fix
Switchboard adds a validation rule that checks the shipping address at checkout and stops the order before it exists, with a message you write ("We can't ship to PO boxes. Please use a street address."). Set it up in the admin in a few minutes: pick the address-block rule, choose PO box detection, write your message, activate. No code.
What to know
The rule runs as a native Shopify Function inside checkout, on every plan. It catches the standard PO box formats; a customer determined to disguise an address can still mistype one, so keep your carrier's rejection flow as the backstop, not the front line.
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