I think all the agencies charge this (the MK fee). Some just have a separate Magic Kingdom resort pricing scheme. I wouldn't worry about this too much. Happy Limo charges $8.00 more for MK resorts and Tiffany charges $5.00 more for MK resorts. (Although this will likely read $8.00 when and if they get around to updating the website to reflect the price increase from Disney). I agree that it would be better if Payless just had a separate pricing structure for Magic Kingdom resorts (like some other companies), but at least they did mention the cost. Perhaps if people mention that this is confusing when booking they will make it more clear.
However, all that being said, the $70.00 special rate on the front page says that it is for "all Disney Destinations" and there is absolutely zero mention of MK resorts on that entire page. I would not expect that you should have to pay the extra $8.00 if you are using the special. If they try to charge you and you point this out to them and they still want to charge you, then I would not go with them. Charging an additional fee for some Disney destinations (MK resorts) when the special is for all Disney destinations (with no asterisk or anything else indicating conditions) would be unprofessional.