Customer. is trying to use a credit card to pay his invoice. He already paid half in cash. QB will not accept payment. Tried 3 cards.

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When a customer's credit card is declined, they can contact their card issuer and confirm the exact reason why they chose to decline their transaction from taking place. There's a series of reasons a decline can occur.
 

Here's some things you can check:
 

• Use your customer's address from the US USPS website.
• Confirm there's no special characters or spaces in any data fields while entering their card details.
• Make sure the card number is correct. Most card numbers are 16 digits long. American Express cards are 15 digits long.
• In addition to the card number, you need to enter a verification code. For most cards, like Visa, Mastercard, and Discover, their cards have three digit codes on their backsides. For American Express, it's a 4 digit code on the front of your customer's card.

If you're trying to process a business credit card, for example, some of these cards require additional commercial codes to process transactions successfully. You can get in touch with your Customer and ask them to verify if their employer requires a special commercial code for purchases.
 

In the event your customers need to use special commercial codes for their credit cards, go to the Merchant Service Center and update your industry type. This will enable the account to process commercial codes.
 

Here's how to update your industry type:
 

1. Sign in to the Merchant Service Center.
2. From your top menu bar, go to Account, then Account Profile.
3. In the Account Holder/Business Information section, click Edit.
4. Enter your last for digits of the business's Tax ID number, or your SSN.
5. Hit Submit.
6. Choose an appropriate Industry Type.
7. Specify a Reason for this account change.
8. Select Submit.

You can also check your transaction log in the Merchant Service Center, under Activity & Reports, to verify a transaction's status and other information about declined, approved, or pending transactions.
 

In the event you need to process flex spending (FSA) cards, you can review our Accept flexible spending cards article.

I've also included a detailed resource about working with declined credit card transactions which may come in handy moving forward: Fix customer's declined credit card payments
 

If there's any questions, I'm just a post away. Have an awesome Tuesday!