Flattring without funds is pretty pointless as you should give to show your appreciation. But I think what you're asking for is already there. You can login to flattr and in there "Add funds" and use lots of systems to reload your flattr account - and also use money you got from people flattring you - which is what you're asking for if I understand right. It's hard to find if you never click on "add funds" of course ;-) More

1 Flattr microdonation