Does anyone have any advice on which of the many commercial users manuals available would be a good one to buy?

Also, has anyone had any problems returning to the store after the PayPal transaction?

I think you will need to clarify what type of "user manual" you are wanting. :) Since you posted in the osCommerce area is that what you are looking for?

Return options would depend on the vendor you purchased from and not so much weather you used PayPal or not.