All the files in the "for_upload" directory need to be uploaded to your site. The "How to Install" says that. I don't consider the includes/modules/pages/checkout_one/jscript_square_WebPay,php to be "extra".

I look at it as something that should have been included in the original Square mod rather than instructions saying to add the file IF you used OPC. Having it part of the mod will keep a lot of folks from making the mistake of not including this file when they start using OPC.

Not mentioned is the need to change the redirect URL at Square.

I put a method that i use in the second post. This method will have your site down for the least amount of time and get the steps at Square AND your site taken care of safely.