I've been thinking (despairing) about this problem a lot in recent days as BTC and all other cryptos have not just been on a steady decline but dropping off cliffs in the span of 1-2 hours a few times in the last five days:
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If the customer hasn't cryptocurrency on an exchange or wallet already, the exchange process from FIAT can be delayed for several reasons, especially with exchanges regularly requiring repeated verification of identity or locking first transfers for up to 72h before releasing them.
Many exchanges (especially exchange services that use credit card/ATM) obfuscate commissions & transfer fees and/or exchange fiat to crypto at a worse rate for additional profits, which can lead to insufficient funds being sent if these are deducted from the amount transferred instead of added on top. You can compare to the actual exchange rate by entering for example
'100usd in BTC' into Google, they pull data Coinmarketcap at a three minute delay.
Purchasing a
legitimate stablecoin like USDC and converting that to BTC etc. at no extra fee (most exchanges offer this) once you are ready to pay for your order can circumvent some of these pitfalls (anonymity of crypto transfers is an entirely different rabbit hole so I'm not considering that here).
While annoying, it's ultimately an insignificant extra step compared to the time and effort spent on planning your cycle and purchase ahead of time.