first, that 3.9% fee is not the foreign transaction fee i'm talking about. that is a fee that paypal (not your card) is charging you. they typically charge something like this when you pay F&F with CC. it is partially to offset the CC merchant fees they incur. and partially for profit. it also depends on the country when you're doing an intl payment. it's pretty much like paying the "G&S" fee, but the sender experiences it on his end. if you did G&S instead of F&F, you wouldn't see it, but that same fee (probably identical amount) then comes out of the recipient's take.
1.5% cash back? do you mean chase freedom unlimited? (
"Chase gives YOU the Freedom to get yourself into unlimited debt!"®)
if so, fyi that card does charge a 3% foreign transaction fee. and i'm almost certain that this transaction would incur one since you're probably sending it to a non-US PP account. even if you get +1.5% cash back, with the -3% fee, you're still at -1.5%.
so you're better off using a card that doesn't have a foreign fee. typically the best you can do then is 1% cash back.
also btw, paying in foreign currency is not a good predictor of if you get hit with this foreign transaction fee. likewise, paying in USD instead will not save you from it. it's not that simple. you get hit with that fee pretty much if a foreign bank is somewhere in the transaction. like i said, you basically can't know it for sure ahead of time. they don't give us peon consumers tools to simulate or trace a transaction.
so when in doubt, don't get greedy with those CC points, especially on a big transaction. use your basic-ass, no-foreign-transaction-fee card and settle for 1% cash back.