All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is permissible for these companies to give back a certain percentage to the buyer who signed up for these offers, as the reality is reducing the price, and it is permissible for you to leave this money with them so that they would invest it in what is permissible and according to the Sharee’ah conditions of investments.
However, this does not happen. By referring to the method of these offers in the site that you mentioned, it is clear that this money is put in the account of the buyer as non-taxable savings and it has interest until the buyer becomes able after a period of time to benefit from these accumulated amounts in paying the expenses of the university and the like. The method of saving with an interest is not permissible as this is forbidden Riba (interest and/or usury). Therefore, it is not permissible to participate in such savings.
Allaah Knows best.