Cart subtotal way too high, zip code going in product quantity box

Technical Q&A

Topic summary

A store owner reports a critical cart bug affecting checkout. When customers enter a zip code in the shipping/tax estimator, the cart subtotal incorrectly jumps to hundreds of thousands or millions of dollars.

Technical Details:

  • After entering a zip code and refreshing the page, the zip code value appears in the product quantity field instead
  • The price returns to normal when only 1 item remains in cart
  • Issue occurs specifically during the estimate shipping/tax step

Impact & Context:

  • The bug is causing lost sales as customers are concerned about the incorrect amounts
  • Store owner has confirmed no apps or promotions are affecting the cart price estimator
  • Uncertainty exists whether this is a Shopify platform issue or theme-specific problem
  • A screenshot has been provided showing the issue
Summarized with AI on November 18. AI used: claude-sonnet-4-5-20250929.
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Currently on my store when you go to the cart, you can enter a country/state/zip to estimate shipping and tax.

After you enter the zip code, the subtotal jumps to several hundred thousand dollars or even several million dollars.

It appears when you refresh the page, the zip code entered shows up as the product quantity.

When you proceed to checkout with just 1 item in the cart, the price goes back to normal, but for some reason in the cart the subtotal is way wrong and this is definitely costing me sales as customers are concerned of the incorrect amount.

I do not have any apps or promo’s that would affect the cart price estimator, I am not sure if this is a shopify issue or a theme issue.

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