Transaction posted:
Looks like PayPal used the correct currency after all, but according to MC's currency tool:
March 5th: 1 GBP = 1.526601 USD
March 6th: 1 GBP = 1.523299 USD
March 9th: 1 GBP = 1.511999 USD
DC's currency rate = 1.523571 USD
I'm guessing they used the 6th's rate + a markup. Not sure if an extra $0.000572 is worth complaining about since PayPal wanted to charge me $1.562548 per GBP.