What can I do if I submitted a paper believing a journal to be free for authors, and after acceptance I'm asked to pay a fee? Reading about a journal A, I understood that authors can submit papers free of charge to this journal. I have submitted a paper and received a letter of acceptence with a bill. The editor said that I have to pay print charges in order to publish my paper.

What should I do?

Can I withdraw my paper?
Yes, you may tell the editor that you were unaware of the author fees, and that since you can't pay it you are going to withdraw the paper. (They may agree to waive the fees, but if they don't, you certainly have the right to withdraw your paper and submit it elsewhere.)

In the future, you should check a journal's own instructions to authors rather than (or in addition to) third-party sites, which don't always have complete or up-to-date information about the journal.