You can invest whichever way the player allows, but yes, Paypal is way too risky. They consider money used for gambling related events against their terms which nullifies those transactions.
As for Cake, here's a Sticky that explains it well:
http://forum.neverbeg.com/showthread.php?t=104032
Heeb has evolved into a stake consisting of multiple games without all of the buy-in's provided. i.e. If I stake $65 to play 10 $6.50's, it's a series. If I staked $30 to do it, it'd be a heeb because completion depends on rolling over profit.