[vim] How to add text at the end of each line in Vim?

I have <M-DOWN>(alt down arrow) mapped to <DOWN>. so that I can repeat the last command on a series of lines very quickly. with this mapping I can:

A,<ESC>

And then hold alt while pressing down repeatedly to append the comma to the end of each line.
This works well for me because it allows very good control over what lines do and do not get the change.
(I also have the other arrows mapped similarly to allow for easy repeating of .)

Here's the mapping line to paste into your vimrc:

map <M-DOWN> <DOWN>.