Mangagamer has discussed Paypal payment in the past. Currently they're not compliant with Paypal's standards, therefore Paypal refuses to deal with them. To become fully compliant, Mangagamer would have to remove all free adult content from its shop, including demos and sample HCG. Mangagamer doesn't want to do that, therefore no Paypal.
This is kind of the issue with a small company holding a monopoly on distribution. It limits options, limits exposure, and forces the company to throw resources at a distribution system that might be better spent somewhere else...like marketing. Even with the success of Steam, Mangagamer still hasn't solved the basic problem of offering solid distribution for their titles at launch that everyone can be happy with.