Once logged in to your Paypal account, click on the gear (settings) in the navbar, then on the "Payments" tab, followed by "Preapproved Payments".
Next click on "Set Available Funding Sources" and here you'll be able to see the cards in your account. If you click on "Conversion Options" next to your Shakepay card, you'll be able to switch the way Paypal converts currencies on your card.
It's usually cheaper to make the currency exchange through your issuer than through the merchant. This is called Dynamic Currency Conversion and the merchant (Paypal) usually charges higher fees than the 3% that's charged from your issuer (Shakepay). In a case where you're adding USD from a USD account, you will not be paying any FX on the transaction.
Select the "Bill me in the currency listed on the seller's invoice" and that should do it!.