I know Phortune is a prototype, but this is a pretty simple issue, and it has a pretty simple fix.

Basically all Paypal transactions are sent to the sandbox site regardless of the mode set on the provider.
Basically you can't make a payment to paypal, at all.

Repro:
site.com/phortune/merchant/
Create a new paypal merchant account,
Fill in all the required info, and set the mode to Live,
Make a new initiative in Fund, try and back it using paypal,
You will be sent to sandbox.paypal.com and get an error.

The problem is this line: https://secure.phabricator.com/diffusion/P/browse/master/src/applications/phortune/provider/PhortunePayPalPaymentProvider.php;e2685a248b8c3071ab5619506ee7d2c2171f03c5$351

It should omit the sandbox part if the provider is in live mode.

phabricator e2685a248b8c3071ab5619506ee7d2c2171f03c5 (Mon, Apr 4)
arcanist 8701e6c1f31ab7e3b7f35482d4bc670fde32ec99 (Sat, Apr 2)
phutil 3024f0a4908be46ddec0198bf6e59e58a5611e2b (Thu, Mar 31)