Our system updates the forex spot rate on a daily basis, based on the actual rate that day, converting from USD to the desired currency.

All of our billings occur in USD, therefore the rate applied will always be the spot rate on the date of billing.  If a you're on credit terms, and you're using PayPal to settle your bill on a weekly basis, there will be slight changes to the amounts billed if they translate on the date of billing vs. the date of payment.