Stripe has two currencies:
Invoice currency - As presented to the customer.
Bank currency - Payment made to your bank account.
They are independent of each other. Conversion is made within Stripe.
The bank currency is set in the Stripe dashboard.
Kashflow has a similar setting, so that they agree with each other.
The Invoice currency is set by the invoice itself. If you invoice in Euro's for the convenience of your Euro customers, then that's what they should be presented with.
However, Kashflow is using the bank currency for this. My Euro customers are being asked to pay (in my case) GBP. So now they are having to pay the conversion fees, which they did not agree to.
KashFlow Support wouldn't accept this, and told me to file it as a feature request.