Why the wrong amount arrives
Four causes, in order of frequency.
The wallet deducted the fee from the amount
Overwhelmingly the leading cause. The customer entered the exact figure, and the wallet subtracted the network fee from it instead of adding it on top. The shortfall equals the fee, which is why a payment short by exactly a dollar on TRON is almost always this.
The exchange withdrawal fee
Same effect from a different source. A customer withdrawing directly to your invoice has the exchange’s fee deducted, which is frequently larger than the network fee.
The rate window expired
The invoice locked a rate, the customer paid later, and the crypto amount no longer matches the fiat value.
Manual entry
Typing the amount rather than scanning the QR code.
Why overpayment happens
Overpayment has narrower causes: rounding up, entering the amount before the rate refreshed, or paying two invoices with one transfer.
What a gateway does
Most providers apply a tolerance: a shortfall below a small threshold, often a fraction of a percent, is accepted and the order is credited. This exists precisely because the fee-deduction case is so common and so blameless.
Above the tolerance the invoice stays unpaid with the received amount recorded, and the decision passes to the merchant.
What to do about it
The merchant’s options on underpayment
Three, and the right one depends on the gap and on what was sold.
Request the difference
Send a top-up invoice for the shortfall. Reasonable when the gap is meaningful and the customer is reachable.
Credit anyway
Absorb a small shortfall and fulfil the order. Frequently the cheapest option once support time is counted: chasing two dollars costs more than two dollars.
Refund and cancel
Return what arrived and close the order, following your refund policy including who pays the return fee.
Decide the thresholds in advance
Whichever you pick, decide the thresholds in advance. Handling each case individually is how support queues grow.
Overpayment
Credit the order and return the excess, or credit the excess to the customer’s account where you have one.
The return raises the same address question as any refund: ask where to send it rather than assuming the sending address is safe, since it may belong to an exchange.
Preventing most of it
Three settings at the checkout, and together they remove the majority of cases.
A QR code with the amount embedded
The customer scans and the wallet fills in the figure, eliminating manual entry and rate confusion.
A visible warning about fee deduction
One line telling the customer to ensure the recipient receives the full amount. This is the cheapest fix available for the leading cause.
A clear countdown
Customers who see the window are less likely to pay outside it.
What else reduces it
Beyond that, a cheap network reduces the size of every fee-related shortfall, and stablecoins remove the rate-movement cause entirely. The setup guide covers the wider configuration.