on the shipping charges - that is the thing that makes or breaks a sale for me. And some of these on-line sites make it almost impossible to know what those charges are until after you hit the "submit" button. How a site computes their shipping charges can be buried in the fine print, or written so it is absolute gibberish.
Which is another reason to pay with a credit card - it gives you some leverage. Case in point, I ordered a tool from an on-line site, and under Shipping Charges it said $7.50. Insane gouging for something that weighed 3 ounces, but I needed the tool and they are the only source, so ... when I got the invoice with the tool, the shipping charges had mysteriously increased to $12.45 because I had it shipped to a P.O. box instead of my home (where it would have sat outside all day for anyone to walk off with). Multiple e-mails were ignored, until I called my credit card company and slapped a hold on the charge. Then, surprisingly, came a response from said company, admitting that the shipping charges shown when you order could be misleading, but their policy was clearly spelled out elsewhere on the website. Maybe so, but it left a very negative impression on me and I doubt I will buy from them again unless they are the sole source for something.