Stripe claim that their exchange rate is "generally... the mid-market rate", but recently for USD/GBP we have been getting rates in excess of 1% below this. Support say this is due to the specific timings of sales and there are no hidden fees, but the rates we get are often below even the lowest mid-market rate of the week. This apparently disingenuous practice has been happening consistently all year, totalling perhaps £1k.

7 Jan - 0.7782 31 Dec - 0.7776 24 Dec - 0.7780

Over this time period, the rate listed here (and on other sources) has been typically above 0.785: https://wise.com/tools/exchange-rate-alerts/?fromCurrency=USD&toCurrency=GBP