I was able to pay my sample invoice but when my customer try’s to pay her invoice via credit card or bank transfer it gets declined?

It's good to see you here in the Community space, nsomerville. I recognize the importance of processing payment invoices in QuickBooks Online (QBO).

When your card is declined during a payment transaction, it can be due to the following reasons:

• Invalid account number
• Expired credit card
• Overlimit or past due
• General decline, no other information provided by processor
• Issuing bank will not electronically authorise the credit card
• Billing Address in QuickBooks Online does not match the address on your statement

If you think none of the above-mentioned is applicable to you, please contact your credit card issuer for the exact reason.

Let me add this article as your reference for more answers to why your customer's credit card may be declined: Why is my customer's card declined?

Should you need additional concerns regarding receiving invoice payments in QBO, feel free to post them in the Community space.