How to undo my paid authorship?
There is no guarantee that you succeed, but you can still try.


Address the legal side of the issue.
Read the terms of your agreement carefully. If it is possible to abide by the terms of contract and have you name dropped, act accordingly. If not, seek for legal advice. After all, the contract might be not 100% clean according to your country's law or international laws, and you could hypothetically declare the contract for void in this case. If the contract has not been written on paper but is a handshake deal, check for the legislation of your country again about handling such cases.
Address the academic side of the issue. For that, read the terms of the venue(s) the paper(s) went to. Check whether these terms request that each author has to contribute in a substantial way. If such a phrase is not there, you have less chances. If such a phrase is there in some form, you have more chances. In each case, write to the editor(s) and supply a corresponding justification for why your name is to be dropped from the author list.