I got into one of those sites. Sounded okay as one was having a sale for .98 or maybe 1.98. Very reasonable. Of course, they assumed I also wanted a membership at the rate of 24.98/month. I talked to them several times, making notes of date, time, and person I spoke with. Then I called the credit card company. All charges were credited the following month. The outfit called itself peoplefinder.
I can see a convenience fee, but keep it reasonable and be honest about what you are really planning to charge.