Crypto to fiat payments are currently settled in USD stablecoins (see list of supported currencies), pegged at 1:1. For non-USD payments, we use a provider to get rates:
USD payments: 1:1 pegged to USD stablecoins
EUR payments: EUR to USDC/T rate as provided by Bitstamp
For other currencies: USD mid-market rates provided by Open Exchange Rates
There's no slippage for crypto to fiat payments.