Good point. I pay a roughly 3% commission to PayPal for my tiny side business. I think I do get something. The biggest thing, which I also can't quantify, is probably that I get X percent more sales because people are confident that PP will somehow magically take care of them if I turn out to be a screwball. All it takes is a couple more sales, to completely pay for the commission.
Also, I could knock myself out trying to find a cheaper service, or I could spend that time designing the next product.
I wasn't even including the ~3% payment commission.
Back in the day before eBay required everything to go through Paypal, I was happy to give a discount for anything but Paypal: COD, cash-in-mail or money order. I earned some add'l sales from the voluntarily and involuntarily un/under-banked.
Now eBay is going to automatically charge Canadian sales taxes on my Canadian sales... even though I would be exempt myself as a small supplier. Ugh.
Also, I could knock myself out trying to find a cheaper service, or I could spend that time designing the next product.
Don't know if a similar calculus applies to eBay.