Mingw-w64 is almost entirely made by volunteer. Only two employees are paid to contribute to mingw-w64 and even then it's not full-time: we estimate the paid time spent on the project matches 0.5-employee. All other contributions are done on free time.
There are many evolutions and features planned but resources are scarce. In particular, if mingw-w64 saves you from the steep cost of proprietary toolchains, think about contributing a bit; you will also most likely quickly benefit from the improvements.
More than through money, the best way to help the project is to invest man-time in it. The list of tasks on the page about contributing is the best place to start and if you are an employer with trainees, several of these tasks will make good internship.
With the current organization and funds of mingw-w64, it wouldn't make sense to try to pay a developer to spend time on specific topics. Instead, the money will be spent on hardware (testing servers, Windows licenses, phone, tablets, …) and meeting expenses (setting up hackatons and paying for travel expenses as some major mingw-w64 developers live very far from others).
Currently, only Paypal is available:
Below is a list of donors, starting from January 2015. We'd like to list all our donors but we value your wishes; please state whether you'd like to have your name, pseudonym or company on the website.
There is no facility at the moment to receive hardware. These donations will have to be handled on a case-by-case basis (mostly to decide where to ship).