I don't think they subsidize through other rates. They tend to lose money every year, and their rates for packages under 1 lb are significantly better than UPS or FedEx.
The USPS can't increase their domestic prices but there is good indication that they are charging more to offset this. There are complaints in this thread even about the cost of shipping abroad, as international shipping is not regulated like domestic postage rates if I remember right. So the USPS uses outbound international shipments as a cost center to make up for the subsidy to China.
The article states there is a ~$500M subsidy to China through UPU rates.