While adding text to PHP files, I found apostrophes (') needs to be written with backslash (\') so as to not interfere with PHP's syntax. However, that is not likely to help you as the customer's input should be modified.
There are other input fields in default osCommerce that does not face such problems (product description field in admin, for example), so I think the addon's implementation is wrong. I do not understand PHP beyond following instructions to edit and install addons, so I cannot help.