If you already have a merchant account e.g. take card payments on site? There's the option of going it alone.
This is what the college I work at did, we created our own system from scratch. All be it probably very over the top for what you need, as this provides a POS terminal for several locations around the college site and has custom integration to any and every service that we need. Haggled over our fees with the merchant and got ourselves a substantially cheaper deal than going with ParentPay etc.
If you feel that the cost of going with these companies is too much then it's worth looking at the huge initial outset for the cost savings further down the line.
However, in the proccess of setting our own up we went to look at a school that was using WisePay, and their feedback was that it was fantastic. Parents/staff loved using it and couldn't speak ill of it.