Changing the bibliography heading style with `apacite` and the `letter` document style

I am writing a letter with an APA-style bibliography, and I want to center the bibliography heading. There are no hooks in apacite for controlling its formatting, so I’m going to have to renew section (which isn’t defined in the letter environment, but which I am defining for the body anyway). So I have written this code:

documentclass[12pt]{letter}

usepackage{microtype}          % better spacing
usepackage[margin=1in]{geometry} % nice margins
usepackage[utf8]{inputenc}     % utf-8 encoding
usepackage{block}              % left-align everything
usepackage[natbibapa]{apacite} % citations
usepackage[english]{babel}     % correct line breaks
usepackage{setspace}           % double spacing

%addto{captionsenglish}{renewcommand{refname}{}}
newcommand{section}[1]{{bf #1}hfillbreak}

begin{document}
begin{letter}{}

bibliographystyle{apacite}
doublespacing
newcommand{newblock}{}
renewcommand{section}[1]{begin{center}#1end{center}}
bibliography{bibliography.bib}
singlespacing

end{letter}
end{document}

But instead of centering my header, it does this:

What does this mean? How do I prevent this?

renewcommand{section}[2]{begin{center}#2end{center}}