Here are some more details on why you may have underpaid:
1. Paying the fiat value of the invoice (USD, EUR..) instead of the bitcoin amount due
Bitcoin exchange rates often differ among wallets and services. This means that bitcoin amounts denominated in fiat (USD, EUR..) in your wallet or account service could be less or more than the actual amount needed to pay the BitPay invoice.
If you choose to manually enter the invoice amount in your wallet before sending, always enter the exact bitcoin amount listed, as indicated in green in the screenshot below.
2. Using a bitcoin exchange to make the payment
Bitcoin exchanges typically deduct fees from any outgoing payments. They also use different bitcoin exchange rates than the ones used on BitPay invoices.
We recommend using a bitcoin wallet on a mobile phone which will allow you to pay an invoice by simply scanning the QR code displayed on the BitPay invoice. This ensures you are sending the right amount of bitcoin.
You can for instance use the BitPay app, the wallet developed by BitPay which is available on all platforms.
3. Not including a fee in the payment amount
Check your wallet client settings to ensure that a standard miner fee or transaction fee amount (we recommend 0.0001 BTC) is included in your payment.
We recommend that you use BitPay's wallet. Its dynamic fee calculation feature which ensures you only send the minimum fee required in order to get a fast confirmation on the bitcoin network
How To Resolve An Underpaid Invoice Issue
To resolve an underpayment issue, you can reach out to the merchant in order to request a refund. Be sure to provide the merchant with a bitcoin refund address.
Marketplaces or ewallets usually accept underpayments and issue instead store credits.