If you are using our previous version of embedded checkout, please refer to the old multiple NFT purchases guide
You can enable users to purchase multiple NFTs in a single transaction using either CrossmintEmbeddedCheckout or CrossmintPayButton: 
<CrossmintProvider apiKey="_YOUR_CLIENT_API_KEY_">
  <CrossmintHostedCheckout
    lineItems={[
      {
        collectionLocator: "crossmint:_YOUR_COLLECTION_ID_",
        callData: {
          totalPrice: "0.001",
          _quantity: 1, // matches your contract's parameter name
        }
      },
      {
        collectionLocator: "crossmint:_YOUR_COLLECTION_ID_",
        callData: {
          totalPrice: "0.002", 
          _quantity: 2,
        }
      }
    ]}
  />
</CrossmintProvider>
For external EVM contracts registered in Crossmint Console, ensure the attribute name in callData matches the parameter name in your mint function. For example: If your mint function has the signature: mintTo(address _to, uint256 _amount) then use _amount instead of quantity. See our Register External Collection guide for details on registering your contract.
All line items must use the same collectionLocator and be on the same blockchain.

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FAQ